dsa: Add MODULE_LICENSE to tag drivers

All the tag drivers are some variant of GPL. Add a MODULE_LICENSE()
indicating this, so the drivers can later be compiled as modules.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/dsa/tag_mtk.c b/net/dsa/tag_mtk.c
index 23210a6..7ecafb5 100644
--- a/net/dsa/tag_mtk.c
+++ b/net/dsa/tag_mtk.c
@@ -106,4 +106,5 @@ const struct dsa_device_ops mtk_netdev_ops = {
 	.overhead	= MTK_HDR_LEN,
 };
 
+MODULE_LICENSE("GPL");
 MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_MTK);