net: dsa: change tag_protocol to an enum

Now that we introduced an additional multiplexing/demultiplexing layer
with commit 3e8a72d1da ("net: dsa: reduce number of protocol hooks")
that lives within the DSA code, we no longer need to have a given switch
driver tag_protocol be an actual ethertype value, instead, we can
replace it with an enum: dsa_tag_protocol.

Do this replacement in the drivers, which allows us to get rid of the
cpu_to_be16()/htons() dance, and remove ETH_P_BRCMTAG since we do not
need it anymore.

Suggested-by: Alexander Duyck <alexander.duyck@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6060.c

@@ -258,7 +258,7 @@ static void mv88e6060_poll_link(struct dsa_switch *ds)
 }
 
 static struct dsa_switch_driver mv88e6060_switch_driver = {
-	.tag_protocol	= htons(ETH_P_TRAILER),
+	.tag_protocol	= DSA_TAG_PROTO_TRAILER,
 	.probe		= mv88e6060_probe,
 	.setup		= mv88e6060_setup,
 	.set_addr	= mv88e6060_set_addr,