ath9k: fix ps-poll responses under a-mpdu sessions

When passing tx frames to the U-APSD queue for powersave poll responses,
the ath_atx_tid pointer needs to be passed to ath_tx_setup_buffer for
proper sequence number accounting.

This fixes high latency and connection stability issues with ath9k
running as AP and a few kinds of mobile phones as client, when PS-Poll
is heavily used

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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

@@ -2186,14 +2186,15 @@ int ath_tx_start(struct ieee80211_hw *hw, struct sk_buff *skb,
 		txq->stopped = true;
 	}
 
+	if (txctl->an)
+		tid = ath_get_skb_tid(sc, txctl->an, skb);
+
 	if (info->flags & IEEE80211_TX_CTL_PS_RESPONSE) {
 		ath_txq_unlock(sc, txq);
 		txq = sc->tx.uapsdq;
 		ath_txq_lock(sc, txq);
 	} else if (txctl->an &&
 		   ieee80211_is_data_present(hdr->frame_control)) {
-		tid = ath_get_skb_tid(sc, txctl->an, skb);
-
 		WARN_ON(tid->ac->txq != txctl->txq);
 
 		if (info->flags & IEEE80211_TX_CTL_CLEAR_PS_FILT)