rt2x00: Ensure TX-ed frames are returned in the original state.

rt2x00: Ensure TX-ed frames are returned in the original state.

Recent changes to the TX-done code of rt2x00 resulted in TX-ed frames not
being returned to mac80211 in the original state, and therefore with
insufficient headroom for re-transmissions.

Fix this by reverting the changes done and by ensuring we remove the inserted
L2pad by moving the header backwards instead of the data forwards.

At the same time also make sure that the rt2x00queue_remove_l2pad will not
move any memory when a frame has no data at all.

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