rt2x00: Move intf_work to mac82011 workqueue

ieee80211_iterate_active_interfaces() no longer acquires the
RTNL lock which means the intf_work handler can be safely
used from the mac80211 workqueue again.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>