ath9k: do not link bf_next across multiple A-MPDUs

This might trip up tx completion processing, although the condition that
triggers this should not (yet) occur in practice.

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

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

@@ -1951,7 +1951,9 @@ static void ath_tx_txqaddbuf(struct ath_softc *sc, struct ath_txq *txq,
 			if (bf_is_ampdu_not_probing(bf))
 				txq->axq_ampdu_depth++;
 
-			bf = bf->bf_lastbf->bf_next;
+			bf_last = bf->bf_lastbf;
+			bf = bf_last->bf_next;
+			bf_last->bf_next = NULL;
 		}
 	}
 }