mt76: do not store aggregation sequence number for null-data frames

Fixes a rare corner case where a BlockAckReq might get the wrong
sequence number.
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/tx.c

@@ -96,7 +96,8 @@ mt76_check_agg_ssn(struct mt76_txq *mtxq, struct sk_buff *skb)
 {
 	struct ieee80211_hdr *hdr = (struct ieee80211_hdr *) skb->data;
 
-	if (!ieee80211_is_data_qos(hdr->frame_control))
+	if (!ieee80211_is_data_qos(hdr->frame_control) ||
+	    !ieee80211_is_data_present(hdr->frame_control))
 		return;
 
 	mtxq->agg_ssn = le16_to_cpu(hdr->seq_ctrl) + 0x10;