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

drivers/net/wireless/ath/ath9k/mci.c

@@ -20,7 +20,7 @@
 #include "ath9k.h"
 #include "mci.h"
 
-u8 ath_mci_duty_cycle[] = { 0, 50, 60, 70, 80, 85, 90, 95, 98 };
+static const u8 ath_mci_duty_cycle[] = { 0, 50, 60, 70, 80, 85, 90, 95, 98 };
 
 static struct ath_mci_profile_info*
 ath_mci_find_profile(struct ath_mci_profile *mci,