leds: cpcap: Convert to global non-reentrant workqueue

leds: cpcap: Convert to global non-reentrant workqueue

schedule_work puts work items into a global workqueue that is
not single threaded.  If the work item is running when a
second schedule_work is called, the second one could be
immediately scheduled onto the second cpu while the first
one is running, breaking the synchronization expected in
cpcap_brightness_work.

Fix by converting schedule_work to queue_work(system_nrt_wq),
which puts the work on a global single-threaded workqueue.

Change-Id: Ieba89e0353b86f11350cb37552afbce5abe87088
Signed-off-by: Colin Cross <ccross@android.com>