ar9170: remove unneeded flush_workqueue()

cancel_delayed_work_sync() and cancel_work_sync() are already being
used therefore already waiting for all pending work by the driver
to have been completed, no need to flush the mac80211 workqueue.

Cc: Christian Lamparter <chunkeey@web.de>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>