net: mscc: ocelot: track the port pvid using a pointer

Now that we have a list of struct ocelot_bridge_vlan entries, we can
rewrite the pvid logic to simply point to one of those structures,
instead of having a separate structure with a "bool valid".
The NULL pointer will represent the lack of a bridge pvid (not to be
confused with the lack of a hardware pvid on the port, that is present
at all times).

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