ath: make ath_hw_set_keycache_entry() static

This fixes this sparse warning:

  CHECK   drivers/net/wireless/ath/key.c
drivers/net/wireless/ath/key.c:110:6: warning: symbol
'ath_hw_set_keycache_entry' was not declared. Should it be static?
Signed-off-by: NLuis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/key.c

@@ -108,7 +108,7 @@ static bool ath_hw_keysetmac(struct ath_common *common,
 	return true;
 }
 
-bool ath_hw_set_keycache_entry(struct ath_common *common, u16 entry,
+static bool ath_hw_set_keycache_entry(struct ath_common *common, u16 entry,
 				      const struct ath_keyval *k,
 				      const u8 *mac)
 {