ath9k_htc: make ath9k_htc_tx_aggr_oper() static

This fixes this sparse complaint:

  CHECK   drivers/net/wireless/ath/ath9k/htc_drv_main.c
drivers/net/wireless/ath/ath9k/htc_drv_main.c:441:5:
	warning: symbol 'ath9k_htc_tx_aggr_oper'
		 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/ath9k/htc_drv_main.c

@@ -438,10 +438,11 @@ static void ath9k_htc_update_rate(struct ath9k_htc_priv *priv,
 			  bss_conf->bssid, be32_to_cpu(trate.capflags));
 }
 
-int ath9k_htc_tx_aggr_oper(struct ath9k_htc_priv *priv,
-			   struct ieee80211_vif *vif,
-			   struct ieee80211_sta *sta,
-			   enum ieee80211_ampdu_mlme_action action, u16 tid)
+static int ath9k_htc_tx_aggr_oper(struct ath9k_htc_priv *priv,
+				  struct ieee80211_vif *vif,
+				  struct ieee80211_sta *sta,
+				  enum ieee80211_ampdu_mlme_action action,
+				  u16 tid)
 {
 	struct ath_common *common = ath9k_hw_common(priv->ah);
 	struct ath9k_htc_target_aggr aggr;