rt2x00: rt2800lib: fix BBP1_TX_ANTENNA field configuration for 3T devices

The field must be set to 2 instead of 0 for
devices with three TX chains.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>