drivers: pwm_nrfx: Wait until PWM is stopped before restarting it

Change adds waiting until PWM is stopped before restarting it. Without
the change, calling the pwm_nrfx_pin_set function multiple times in
quick succession could cause a race condition causing the PWM to
remain stopped in case it should be restarted.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>