platform: msm: Replace mutex with spinlock in QPNP-PWM driver

Since PWM APIs can be called from atomic context, using mutex locks
lead to BUG() messages. Therefore, replace mutex locks with
spinlocks in the driver.

Change-Id: I69a6939e5c2766e9c6beb5fc55b714771dffb9f6
Signed-off-by: Jay Chokshi <jchokshi@codeaurora.org>