drivers: leds-qpnp-wled: Synchronize brightness_set and worker thread

Synchronize between the brightness_set and worker thread by using
led_access mutex lock from LED class device. This makes sure that
the brightness level does not get changed when the register write
is in progress and the last set brightness level always
gets applied.

CRs-Fixed: 1044717
Change-Id: Ia0d213e36814f867ec7b2a14b432f33a62f1e6e1
Signed-off-by: ansharma <ansharma@codeaurora.org>