ath9k_hw: define antenna diversity group

AR9285 belongs to diversity group 0 and AR9485 belongs to diversity
group 2. Based on the diversity group we configure certain antenna
diversity paramaters such as lna1_lna2_delta and fast diversity
bias values. For AR9485 we have some gain table parameter which
selects the gain table 0/1 for main and alternate antenna

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>