staging: rtl8192u: Remove create_workqueue()

With cmwq, use of dedicated workqueues can be replaced by system_wq.
Removed the dedicated workqueue and used system_wq instead.

Since the work items in the workqueues do not need to
be ordered, increase of concurrency by switching to system_wq should
not break anything.

All work items are sync canceled so it is guaranteed that no work is
running when driver is detached.

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