leds: leds-qpnp: Set PWM channel operational mode explicitly

When LED is changing between fully on and blinking, there is a change
in PWM/LPG channel operational mode. Use pwm_change_mode() API to notify
the mode change to the PWM driver. This way, the operational mode and
the underlying configuration for PWM channel can be updated by PWM driver.
For example, this will help with the following use cases where the mode
can be switched at runtime.
  1) When the LED is fully ON, the LPG channel is in PWM mode.
  2) When the LED is blinking, the LPG channel is in LPG mode.

CRs-Fixed: 2070945
Change-Id: Idd7fc31de085b7b1f97c9bf823dfd49a46164018
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>