leds: Queue screen brightness on the suspend workqueue.

Queueing screen brightness change on suspend workqueue ensures that
userspace initiated changes to screen brightness (especially when
the END key is pressed) so not get triggered while early suspend
is in progress.

CRs fixed: 265531

Change-Id: I923e5a4c9ed498336ca5c61e5b4758be3545b49b
Signed-off-by: Anurag Singh <anursing@codeaurora.org>