ath9k: fix initial sequence number after starting an ampdu session

txtid->seq_start may not always be up to date, when there is HT non-AMPDU
traffic just before starting an AMPDU session. Relying on txtid->seq_next
is better, since it is also used to generate the sequence numbers for
all QoS data frames.
Signed-off-by: NFelix Fietkau <nbd@openwrt.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

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

@@ -856,7 +856,7 @@ int ath_tx_aggr_start(struct ath_softc *sc, struct ieee80211_sta *sta,
 
 	txtid->state |= AGGR_ADDBA_PROGRESS;
 	txtid->paused = true;
-	*ssn = txtid->seq_start;
+	*ssn = txtid->seq_start = txtid->seq_next;
 
 	return 0;
 }