mt76: fix handling full tx queues in mt76_dma_tx_queue_skb_raw

Fixes a theoretical issue where it could potentially overwrite an existing
descriptor entry (and leaking its skb)
Signed-off-by: Felix Fietkau <nbd@nbd.name>

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

@@ -271,10 +271,13 @@ mt76_dma_tx_queue_skb_raw(struct mt76_dev *dev, enum mt76_txq_id qid,
 	struct mt76_queue_buf buf;
 	dma_addr_t addr;
 
+	if (q->queued + 1 >= q->ndesc - 1)
+		goto error;
+
 	addr = dma_map_single(dev->dev, skb->data, skb->len,
 			      DMA_TO_DEVICE);
 	if (unlikely(dma_mapping_error(dev->dev, addr)))
-		return -ENOMEM;
+		goto error;
 
 	buf.addr = addr;
 	buf.len = skb->len;
@@ -285,6 +288,10 @@ mt76_dma_tx_queue_skb_raw(struct mt76_dev *dev, enum mt76_txq_id qid,
 	spin_unlock_bh(&q->lock);
 
 	return 0;
+
+error:
+	dev_kfree_skb(skb);
+	return -ENOMEM;
 }
 
 static int