net: dsa: tag_8021q: create dsa_tag_8021q_{register,unregister} helpers

In preparation of moving tag_8021q to core DSA, move all initialization
and teardown related to tag_8021q which is currently done by drivers in
2 functions called "register" and "unregister". These will gather more
functionality in future patches, which will better justify the chosen
naming scheme.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>