drivers: pwm: pwm_nrfx: Apply workaround for stopping PWM instance

Current implementation of `nrfx_pwm_stopped_check()` doesn't work
as expected when user doesn't provide event handler.
Workaround for that is to use low level function for checking whether
STOPPED event arrived.

The workaround should be removed when `nrfx_pwm_stopped_check()`
will contain needed functionality.

Signed-off-by: Adam Wojasinski <adam.wojasinski@nordicsemi.no>