tmk_core/common/timer.h: Fixing TIMER_DIFF macro to calculate difference correctly...

tmk_core/common/timer.h: Fixing TIMER_DIFF macro to calculate difference correctly after the timer wraps.

Let's go through an example, using the following macro:

If the first timer read is 0xe4 and the second one is 0x32, the timer wrapped.
If the timer would have had more bits, it's new value would have been 0x132,
and the correct difference in time is 0x132 - 0xe4 = 0x4e

old code TIMER_DIFF_8(0x32, 0xe4) = 0xff - 0xe4 + 0x32 = 0x4d, which is wrong.
new code TIMER_DIFF_8(0x32, 0xe4) = 0xff + 1 - 0xe4 + 0x32 = 0x4e, which is correct.

This also gives a chance for a smart compiler to optimize the code using normal
integer overflow.

For example on AVR, the following C code:
uint8_t __attribute__ ((noinline)) test(uint8_t current_timer, uint8_t start_timer)
{
    return TIMER_DIFF_8(current_timer, start_timer);
}
With the original code, it gets translated to the following list of instructions:
00004c6e <test>:
    4c6e:       98 2f           mov     r25, r24
    4c70:       86 1b           sub     r24, r22
    4c72:       96 17           cp      r25, r22
    4c74:       08 f4           brcc    .+2             ; 0x4c78 <test+0xa>
    4c76:       81 50           subi    r24, 0x01       ; 1
    4c78:       08 95           ret
But with this commit, it gets translated to a single instruction:
00004c40 <test>:
    4c40:       86 1b           sub     r24, r22
    4c42:       08 95           ret

This unfortunately doesn't always work so nicely, for example the following C code:
int __attribute__ ((noinline)) test(uint8_t current_timer, uint8_t start_timer)
{
    return TIMER_DIFF_8(current_timer, start_timer);
}
(Note: return type changed to int)
With the original code it gets translated to:
00004c6e <test>:
    4c6e:       28 2f           mov     r18, r24
    4c70:       30 e0           ldi     r19, 0x00       ; 0
    4c72:       46 2f           mov     r20, r22
    4c74:       50 e0           ldi     r21, 0x00       ; 0
    4c76:       86 17           cp      r24, r22
    4c78:       20 f0           brcs    .+8             ; 0x4c82 <test+0x14>
    4c7a:       c9 01           movw    r24, r18
    4c7c:       84 1b           sub     r24, r20
    4c7e:       95 0b           sbc     r25, r21
    4c80:       08 95           ret
    4c82:       c9 01           movw    r24, r18
    4c84:       84 1b           sub     r24, r20
    4c86:       95 0b           sbc     r25, r21
    4c88:       81 50           subi    r24, 0x01       ; 1
    4c8a:       9f 4f           sbci    r25, 0xFF       ; 255
    4c8c:       08 95           ret
Wth this commit it gets translated to:
00004c40 <test>:
    4c40:       28 2f           mov     r18, r24
    4c42:       30 e0           ldi     r19, 0x00       ; 0
    4c44:       46 2f           mov     r20, r22
    4c46:       50 e0           ldi     r21, 0x00       ; 0
    4c48:       86 17           cp      r24, r22
    4c4a:       20 f0           brcs    .+8             ; 0x4c54 <test+0x14>
    4c4c:       c9 01           movw    r24, r18
    4c4e:       84 1b           sub     r24, r20
    4c50:       95 0b           sbc     r25, r21
    4c52:       08 95           ret
    4c54:       c9 01           movw    r24, r18
    4c56:       84 1b           sub     r24, r20
    4c58:       95 0b           sbc     r25, r21
    4c5a:       93 95           inc     r25
    4c5c:       08 95           ret
There is not much performance improvement in this case, however at least with this
commit it functions correctly.

Note: The following commit will improve compiler output for the latter example.