rt2x00: Don't queue ieee80211 work after USB removal

This prevents the rt2x00 driver from queueing ieee80211 work after the  
USB card has been removed, preventing a kernel panic.

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