dsa: add support for Trailer tagging format

This adds support for the Trailer switch tagging format.  This is
another tagging that doesn't explicitly mark tagged packets with a
distinct ethertype, so that we need to add a similar hack in the
receive path as for the Original DSA tagging format.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Tested-by: Byron Bradley <byron.bbradley@gmail.com>
Tested-by: Tim Ellis <tim.ellis@mac.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ethernet/eth.c b/net/ethernet/eth.c
index dae47e7..b9d85af 100644
--- a/net/ethernet/eth.c
+++ b/net/ethernet/eth.c
@@ -193,6 +193,8 @@
 	 */
 	if (netdev_uses_dsa_tags(dev))
 		return htons(ETH_P_DSA);
+	if (netdev_uses_trailer_tags(dev))
+		return htons(ETH_P_TRAILER);
 
 	if (ntohs(eth->h_proto) >= 1536)
 		return eth->h_proto;