rt2x00: Split EEPROM_NIC_TX_RX_FIXED

The 2 bits in EEPROM_NIC_TX_RX_FIXED each influence
a different antenna. We might as well split the
definition and directly read the correct bit.

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