at76c50x-usb: remove unneeded flush_workqueue() at usb disconnect

This driver only uses the mac80211 workqueue and mac80211 requires us to
cancel all work at driver stop. Since we now have the cancels in the right
places at stop() we really don't need to flush the mac80211 workqueue so
remove it.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Acked-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>