rt2x00: Don't report driver generated frames to tx_status()

This adds a new flag for the skb_frame_desc structure which is used to tag
rts/cts frames that are generated by the driver. Through the tag we can
recognize frames we have generated ourselves, so we don't report their tx
status to mac80211.

This patch is based on the original patch by
Mattias Nissler <mattias.nissler@gmx.de>.

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