pwm: Use guards for pwm_lock instead of explicity mutex_lock + mutex_unlock

With the compiler caring for unlocking the mutex several functions can
be simplified. Benefit from that.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/2102fe8189bdf1f02ff3785b551a69be27a65af4.1719520143.git.u.kleine-koenig@baylibre.com


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>