ath9k_hw: define modules to get/set Antenna diversity paramaters

these are the two important modules that will be called by the antenna
diversity algorithm module in the rx. this will continuosly configure
the hardware based on the current diversity status obtained
from the algorithm

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>