ath9k: Implement an API to swap main/ALT LNA's

for the diversity group 2(AR9485) we swap the LNA's of main/ALT antenna
based on alternate antenna's rssi average in comparision with main
antenna's rssi, while for AR9285(antenna diversity group 0)we still
follow the older method of looking at the packet count in 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>