drivers: pwm_nrf5_sw: Clear GPIOTE config only when switching to GPIO

Otherwise a glitch can appear on the PWM output when the GPIOTE channel
is reconfigured (when GPIOTE releases the pin, GPIO takes control and
drives it to the last written state which may be different than that
used recently by GPIOTE).

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