tmk_core/common/timer.h: Improve code generation for TIMER_DIFF* macros

tmk_core/common/timer.h: Improve code generation for TIMER_DIFF* macros

Because of integer promotion the compiler is having a hard time generating
efficient code to calculate TIMER_DIFF* macros in some situations.
In the below example, the return value is "int", and this is causing the
trouble.

Example C code:

int __attribute__ ((noinline)) test(uint8_t current_timer, uint8_t start_timer)
{
    return TIMER_DIFF_8(current_timer, start_timer);
}

BEFORE: (with -Os)

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

AFTER: (with -Os)

00004c40 <test>:
    4c40:       86 1b           sub     r24, r22
    4c42:       90 e0           ldi     r25, 0x00       ; 0
    4c44:       08 95           ret

Note: the example is showing -Os but improvements can be seen at all optimization levels,
including -O0. We never use -O0, but I tested it to make sure that no extra code is
generated in that case.