ath9k: make ath_ant_div_conf_fast_divbias static

  CHECK   drivers/net/wireless/ath/ath9k/recv.c
drivers/net/wireless/ath/ath9k/recv.c:1341:6: warning: symbol 'ath_ant_div_conf_fast_divbias' was not declared. Should it be static?

Signed-off-by: John W. Linville <linville@tuxdriver.com>