USB: appledisplay: Remove deprecated create_singlethread_workqueue

USB: appledisplay: Remove deprecated create_singlethread_workqueue

The workqueue "wq" is involved in controlling the brightness of an
Apple Cinema Display over USB.

It has a single work item(&pdata->work) per appledisplay and hence
doesn't require ordering. Also, it is not being used on a memory
reclaim path.

Hence, the singlethreaded workqueue has been replaced with the use of
system_wq.

System workqueues have been able to handle high level of concurrency
for a long time now and hence it's not required to have a singlethreaded
workqueue just to gain concurrency. Unlike a dedicated per-cpu workqueue
created with create_singlethread_workqueue(), system_wq allows multiple
work items to overlap executions even on the same CPU; however, a
per-cpu workqueue doesn't have any CPU locality or global ordering
guarantee unless the target CPU is explicitly specified and thus the
increase of local concurrency shouldn't make any difference.

The work item is self-requeueing and needs to wait for the in-flight
work item to finish before proceeding with destruction.
Hence, it has been sync cancelled in appledisplay_disconnect().
This also ensures that there are no pending tasks while disconnecting
the driver.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>