rt2x00: Rework rt73 antenna selection

This patch changes rt73 antenna selection to what I believe is the correct way.
It also fixes a small selection bug that switched the antennas by accident.

Signed-off-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>
Signed-off-by: David S. Miller <davem@davemloft.net>