ath9k: configure fast_div_bias based on diversity group

configure fast diversity bias based on the antenna diversity group and
based on main/alt LNA configurations. also configure main antenna and
alternate antenna to gain-table 0 for diversity group 2(AR9485)

Cc: Gabriel Tseng <Gabriel.Tseng@Atheros.com>
Cc: Senthilkumar Balasubramanian <Senthilkumar.Balasubramanian@Atheros.com>
Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>