ath: make ath_hw_keysetmac() static

This fixes this sparse warning:

  CHECK   drivers/net/wireless/ath/key.c
drivers/net/wireless/ath/key.c:70:6: warning: symbol 'ath_hw_keysetmac' 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>