The workqueue "workqueue" has a single workitem(&priv->work) 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.

Workitem is sync cancelled in mc13783_ts_remove() to ensure that there
are no workitems pending when the driver is disconnected.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>