ath9k_hw: config diversity based on eeprom contents

* enable LNA-diversity, fast-diversity for AR9485 based  on
  the value read from EEPROM content
* if antenna diversity/combining is supported, set LNA1 for the main
  antenna and LNA2 for the alternate antenna based on the new diversity
  algorithm

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