ath5k: make ath5k_update_bssid_mask_and_opmode() static

This fixes this sparse warning:

  CHECK   drivers/net/wireless/ath/ath5k/base.c
drivers/net/wireless/ath/ath5k/base.c:569:6: warning: symbol
'ath5k_update_bssid_mask_and_opmode' was not declared. Should it be static?

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>