prevent rounding 0 if HWTIMER_SPEED > 1000000L

The current macros in hwtimer.h expect HWTIMER_SPEED to be < 1000000L, otherwise integer arithmetic will round the result down to 0.
Add a case to prevent that.
Merge pull request #1709 from benpicco/hwtimer_fix

allow HWTIMER_SPEED > 1000000