[PWM] Glitches with duty cycle of 1

[johnnyman727]

PWM output seems to have irregular glitches of the pin dropping low when the duty cycle matches the period (ie when the duty cycle == 1). The glitch does not happen with every cycle but over the course of hundreds or thousands of cycles and not in any sort of pattern (see screenshot of signal capture below).

Additionally, the duration of the glitch is always one tick of the prescaled period.

See the original PWM pull request for more context.