rt2x00: Add kill_tx_queue callback function

provide rt2x00lib the possibility to kill a particular TX queue.
This can be useful when disabling the radio, but more importantly
will allow beaconing to be disabled when mac80211 requests this
(during scanning for example)

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