rt2x00: Add "flush" queue command

Add a new command to the queue handlers: "flush",
this moves the flush() callback from mac80211
into rt2x00queue and adds support for flushing
the RX queue as well.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>