pwm: pwm-qpnp: Ignore changing period setting if it's not requested

It's not necessary to reprogram PWM period setting if only
changing PWM duty cycle.

Change-Id: I366e17a5e2f18c09ab0850d00ec08a35e0f1cb9f
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>