pwm: omap-dmtimer: Drop locking

The pwm driver only provides a single PWM line, so there are no concurrent
calls of the callbacks from different consumers. A single consumer is
expected not to do concurrent calls into the pwm framework. So there is
nothing to serialize and the lock can go away.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>