ath9k: add a missing case statement to ath_ant_div_comb_alt_check

The 'ar9003_hw_antdiv_comb_conf_get' function sets div_group to 2
however that value is not used in 'ath_ant_div_comb_alt_check'.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>