[media] s5p-mfc: Remove deprecated create_singlethread_workqueue

alloc_workqueue replaces deprecated create_singlethread_workqueue().

The MFC device driver is a v4l2 driver which can encode/decode video
raw/elementary streams and has support for all popular video codecs.

The driver's watchdog_workqueue has been replaced with system_wq since
it queues a single work item, &dev->watchdog_work, which calls for no
ordering requirement. The work item is involved in running the watchdog
timer and is not being used on a memory reclaim path.

Work item has been flushed in s5p_mfc_remove() to ensure
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: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>