mt76: mt7915: drop zero-length packet to avoid Tx hang

Hardware wouldn't add LLC-SNAP when skb->data_len is 0, which causes
Tx hang, so add a check to drop this kind of packet.
Tested-by: Bo Jiao <bo.jiao@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7915/mac.c

@@ -942,6 +942,9 @@ int mt7915_tx_prepare_skb(struct mt76_dev *mdev, void *txwi_ptr,
 	int id, i, nbuf = tx_info->nbuf - 1;
 	u8 *txwi = (u8 *)txwi_ptr;
 
+	if (unlikely(tx_info->skb->len <= ETH_HLEN))
+		return -EINVAL;
+
 	if (!wcid)
 		wcid = &dev->mt76.global_wcid;