rt2x00: Remove unused variable frame_control from rt2x00mac_tx.

As additional fallout also remove the also unused variable ieee80211hdr.

Reported-by: Johannes Stezenbach <js@sig21.net>
Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <ivdoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>