Fix a corner case where a residual in the PID could cause a non-zero
pwm request even when the target_temp is zero. (Which could lead to a
firmware "Missed scheduling of next pwm event" shutdown.)
Simplify the logic for suppressing duplicate pwm updates and make sure
a zero target_temp always results in a zero pwm update on the
following set_pwm calculation.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>