leds: leds-qpnp-flash: synchronize LED switch node operation

LED switch node operations are invoked both from LED trigger framework
callback and LED work thread. As work thread and LED trigger framework
operations are asynchronous, a race condition occurs when back-to-back
operations happen. Therefore, moving LED switch node operations from
LED trigger framework callback to work thread to eliminate race condition.

Change-Id: Ia962f3aebda9162e3a35773ef3ee27a4434e6d0c
Signed-off-by: Chun Zhang <chunz@codeaurora.org>