net: dsa: Support prepended Broadcom tag

Add a new type: DSA_TAG_PROTO_PREPEND which allows us to support for the
4-bytes Broadcom tag that we already support, but in a format where it
is pre-pended to the packet instead of located between the MAC SA and
the Ethertyper (DSA_TAG_PROTO_BRCM).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa_priv.h

@@ -191,6 +191,7 @@ void dsa_switch_unregister_notifier(struct dsa_switch *ds);
 
 /* tag_brcm.c */
 extern const struct dsa_device_ops brcm_netdev_ops;
+extern const struct dsa_device_ops brcm_prepend_netdev_ops;
 
 /* tag_dsa.c */
 extern const struct dsa_device_ops dsa_netdev_ops;