ath9k_hw: make antenna diversity modules chip specific

this is necessary to support Antenna diversity and combining in new chip
sets such as AR9485, previously Antenna diversity support is available
only in AR9285

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>