rt2x00: Dump beacons under a different identifier than TX frames.

This allows for specific identification of beacons in the debugfs
frame stream.
Preparation for later differences between dumped TX frames and dumped
beacons.

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>