pwm: Use guards for pwm_lookup_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/28807cb5d9dbce66860f74829c0f57cd9c01373e.1719520143.git.u.kleine-koenig@baylibre.com


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