leds: leds-qpnp-flash: add ordered workqueue in flash LED

When flash LEDs are enabled and disabled back-to-back several times,
issue may arise that work threads to perform enablement/disablement
are trigerred out-of-order. Therefore, introduce an ordered workqueue
to ensure proper execution order.

Change-Id: I729326b3413ea588df3861b1f09d2209176363f6
Signed-off-by: Chun Zhang <chunz@codeaurora.org>