drivers: pwm_nrfx: Refactor pwm_set_cycles to fix on/off issues

This commits refactors implementation of the pwm_set_cycles function
to fix the following issues:
- when a channel was already set with a non-zero pulse width, setting
  cycles for another one required specifying a matching period value,
  even if that value was to be ignored anyway when the channel was to
  be set to constant inactive or active level; due to this limitation,
  it was not possible to e.g. use the LED driver API and turn off a LED
  while another one (within the same PWM instance) was blinking
- the above limitation also applied when a channel was set with a pulse
  width equal to period (duty 100%); even though such channel was not
  in fact using the PWM peripheral, other channels within the same PWM
  instance were forced to use the same period
- after a PWM generation was started for a channel, it was not possible
  to change its pulse width before two PWM periods passed (while it
  should be possible to change it after every period); this was caused
  by a looping mechanism that was unnecessarily activated in the PWM
  peripheral

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>