When RPM=0, ncpx/fan.c disables PWM but it's not enabled even after RPM becomes non-zero.

This bug has been fixed in Fizz branch by https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1096547 but we're looking for a more thorough fix, which may require us to (re)define the interface between common/fan.c and chip/*/fan.c and refactor the fan control system (thermal.c, fan.c, chip/*/fan.c, chip/*/pwm.c) based on it.