PGID_CPU must be updated every time a port is configured or unconfigured
as a tag_8021q CPU port. The ocelot switch lib already has a hook for
that operation, so move the updating of PGID_CPU to those hooks.

These bits are pretty specific to DSA, so normally I would keep them out
of the common switch lib, but when tag_8021q is in use, this has
implications upon the forwarding mask determined by
ocelot_apply_bridge_fwd_mask() and called extensively by the switch lib.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>