staging: most: hdm-usb: Remove create_workqueue()

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

Since there is only one work item per buf_anchor and most_dev and they
do not need to be ordered, increase of concurrency by switching to
system_wq should not break anything.

Both work items are sync canceled before the driver can be
unregistered, to ensure no work item is pending or executing on any
CPU by the time exit path is in flight.

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