USB: ueagle-atm: use system_wq instead of dedicated workqueues

With cmwq, there's no reason to use separate workqueues.  Drop
uea_softc->work_q and use system_wq instead.  The used work item is
sync flushed on driver detach.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>