[media] ad9389b: Remove deprecated create_singlethread_workqueue

The workqueue work_queue is involved in EDID (Extended Display
Identification Data) handling.

It has a single work item(&state->edid_handler) and hence
doesn't require ordering. It is not being used on a memory reclaim path.
Hence, the singlethreaded workqueue has been replaced with
the use of system_wq.

&state->edid_handler is a self requeueing work item and it has been
been sync cancelled in ad9389b_remove() to ensure that nothing is
pending when the driver is disconnected.

The unused label err_unreg has also been dropped.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>