staging: gdm724x: gdm_mux: Remove create_workqueue()

staging: gdm724x: gdm_mux: Remove create_workqueue()

With concurrency managed workqueues, use of dedicated workqueues
can be replaced by using system_wq. Drop mux_rx_wq by using system_wq.

Since there is only one work item per mux_dev and different mux_devs
do not need to be ordered, increase of concurrency level by switching
to system_wq should not break anything.

cancel_work_sync() is used to ensure that work is not pending or
executing on any CPU.

Lastly, since all devices are suspended, which shutdowns the work item
before the driver can be unregistered, it is guaranteed that no work
item is pending or executing by the time exit path runs.

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>