Changes in this commit:
- Limit duty_u16() to 65535 and duty_ns() to the period duration.
- Return 0 for pwm.freq() if the frequency has not been set yet.
- Return 0 for pwm.duty_us16() and duty_ns() unless both frequency and
duty cycle have been set.
- Initialize the pin to PWM at the very end of the constructor, to avoid
possible glitches on the pin when setting up the PWM.
7952694a3c