rt2x00: Make use of ieee80211_free_txskb in tx path

ieee80211_free_txskb should be used when dropping a frame in the device
rx path such that mac80211 knows about this frame being dropped.

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