ath9k: make ath_mci_duty_cycle static

This fixes this sparse warning:

  CHECK   drivers/net/wireless/ath/ath9k/mci.c
drivers/net/wireless/ath/ath9k/mci.c:23:4: warning: symbol 'ath_mci_duty_cycle' was not declared. Should it be static?

Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>