rt2x00: Don't enable broad- and multicast buffering on USB devices

Since rt2x00 USB devices have no chance to know when a beacon was sent
out in AP mode currently all broad- and multicast traffic is buffered in
mac80211 but never sent out at all.

Unfortunately we have no chance in sending the traffic out after a
DTIM beacon due to hw limitations. Hence, instead of never sending the
buffered traffic out better send it out immediately.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Reported-by: Lauri Hintsala <lauri.hintsala@bluegiga.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>