This prevents the rt2x00 driver from queueing ieee80211 work after the  
USB card has been removed, preventing a kernel panic.
Signed-off-by: NSean Cross <sean@chumby.com>
Signed-off-by: NIvo van Doorn <IvDoorn@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>