rt2x00: Split rt61/rt73 antenna selection into RX and TX antenna

Based on investigation of the legacy drivers, I have made the
following assumptions of the antenna setup:
 - R77 is the TX antenna configuration
 - RF2529 fetches default antenna selection from NIC eeprom word

With these assumptions we can change the antenna configuration
to correctly read both antenna setup values and correctly
configure the antenna. And we can now also configure the antenna
for RF2529 without a double antenna.

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>