net: mscc: ocelot: switch from {,un}set to {,un}assign for tag_8021q CPU ports

There is a desire for the felix driver to gain support for multiple
tag_8021q CPU ports, but the current model prevents it.

This is because ocelot_apply_bridge_fwd_mask() only takes into
consideration whether a port is a tag_8021q CPU port, but not whose CPU
port it is.

We need a model where we can have a direct affinity between an ocelot
port and a tag_8021q CPU port. This serves as the basis for multiple CPU
ports.

Declare a "dsa_8021q_cpu" backpointer in struct ocelot_port which
encodes that affinity. Repurpose the "ocelot_set_dsa_8021q_cpu" API to
"ocelot_assign_dsa_8021q_cpu" to express the change of paradigm.

Note that this change makes the first practical use of the new
ocelot_port->index field in ocelot_port_unassign_dsa_8021q_cpu(), where
we need to remove the old tag_8021q CPU port from the reserved VLAN range.

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