ath9k_hw: Use HW cap ATH9K_HW_CAP_ANT_DIV_COMB

Instead of trying to determine whether antenna diversity is
enabled by parsing eeprom data, use ATH9K_HW_CAP_ANT_DIV_COMB
which is already filled at init time.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>