When we remove the active interface, there's no need to continue
sending beacons; doing so would cause a null pointer deref in
ieee80211_beacon_get().  Disable the interrupt in remove_interface
and add a WARN_ON(!vif) in case there are other instances lurking.
Signed-off-by: NBob Copeland <me@bobcopeland.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>