net: wan: x25_asy: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 114928
Signed-off-by: NGustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/wan/x25_asy.c

@@ -324,6 +324,7 @@ static netdev_tx_t x25_asy_xmit(struct sk_buff *skb,
 		if (err != LAPB_OK)
 			netdev_err(dev, "lapb_disconnect_request error: %d\n",
 				   err);
+		/* fall through */
 	default:
 		kfree_skb(skb);
 		return NETDEV_TX_OK;