pwm: Use guards for export->lock instead of explicity mutex_lock + mutex_unlock

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

There is just one caller left for mutex_lock(&export->lock). The code
flow is too complicated there to convert it to the compiler assisted
variant.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/210010f2e579a92476462726e18e0135f6854909.1719520143.git.u.kleine-koenig@baylibre.com


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