The pwm hardware only checks the compare register after a decrement,
so the pin never toggles if tcmp = tcnt.

This happens when a very low duty cycle is requested. Fix it by always
ensuring that tcmp < tcnt.
Signed-off-by: NPeter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: NBen Dooks <ben-linux@fluff.org>