rt2x00: Add antenna setting for RT3070/RT3090/RT3390 with RX antenna diversity support

For RT3070/RT3090/RT3390 with RX antenna diversity support, we must select
default antenna using gpio control way even if we do not turn on
antenna diversity feature.

Seperate the meaning of TX/RX chain and antenna. Some chips use
2x2 TX/RX chain but may have 3 RX antennas or 1x1 TX/RX chain
but may have 2 RX antennas to do antenna diversity.

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