usbnet: allow mini-drivers to consume L2 headers

Assume the minidriver has taken care of all L2 header parsing
if it sets skb->protocol.  This allows the minidriver to
support non-ethernet L2 headers, and even operate without
any L2 header at all.
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/usbnet.c

@@ -324,7 +324,10 @@ void usbnet_skb_return (struct usbnet *dev, struct sk_buff *skb)
 		return;
 	}
 
+	/* only update if unset to allow minidriver rx_fixup override */
+	if (skb->protocol == 0)
 		skb->protocol = eth_type_trans (skb, dev->net);
+
 	dev->net->stats.rx_packets++;
 	dev->net->stats.rx_bytes += skb->len;