drivers: pwm_nrf5_sw: Correctly use all channels provided by generator

Use compare channel 0 in the generator for handling the PWM period.
This way all other channels offered by the generator can be easily
used for handling pulses on particular PWM channels.
So far the driver allowed to configure more than 3 channels for certain
TIMER instances, but since channel 3 was always used for the period,
the generation could not work in such setups.

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