[hm] Extended Fan Voltage Control

Inverted the PWM output to enable the ability to reach 0/256 duty cycle.
Added a new isr that gets turned on if the normal duty cycle is being
asked to go below 1/256.
FanVoltWrite has taken over most of the chore of AnalogWrite. It's doing
some tricks that cost the SMPS range.
I've noticed the output does get jittery as the SMPS goes from
continuous to discontinuous inductor output. Tried about 30 different
revs and couldn't fix the non-linear that happens for about 1/2 volt
output.  Would need much faster sampling to correct. Tweaking
TEMP_OUTADJUST_CNT seems to provide the best compromise.