rt2x00: Implement flush callback

Implement a basic flush callback function, which simply loops
over all TX queues and waits until all frames have been transmitted
and the status reports have been gathered.

At this moment we don't support dropping any frames during the
flush, but mac80211 will only send 'false' for this argument anyway,
so this is not important at this time.

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