Bluetooth: Fix drop of acked packets on ERTM

l2cap_drop_acked_frames() was droping not sent packets, causing them to
be not transmitted.
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/l2cap.c

@@ -1262,7 +1262,8 @@ static void l2cap_drop_acked_frames(struct sock *sk)
 {
 	struct sk_buff *skb;
 
-	while ((skb = skb_peek(TX_QUEUE(sk)))) {
+	while ((skb = skb_peek(TX_QUEUE(sk))) &&
+			l2cap_pi(sk)->unacked_frames) {
 		if (bt_cb(skb)->tx_seq == l2cap_pi(sk)->expected_ack_seq)
 			break;