pwm: sti: Drop driver local locking

The pwm core already serializes calls to .apply(), so the driver local
mutex adds no protection and can be dropped.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/7ad150e40b45d6cb16fee633dcd6390a49a327a1.1750788649.git.u.kleine-koenig@baylibre.com


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