net: usbnet: mark expected switch fall-through

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

Addresses-Coverity-ID: 1077614 ("Missing break in switch")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/usb/usbnet.c

@@ -1527,6 +1527,7 @@ static void usbnet_bh (struct timer_list *t)
 			continue;
 		case tx_done:
 			kfree(entry->urb->sg);
+			/* fall through */
 		case rx_cleanup:
 			usb_free_urb (entry->urb);
 			dev_kfree_skb (skb);