net: dsa: b53: Keep CPU port as tagged in all VLANs

Commit c499696e ("net: dsa: b53: Stop using dev->cpu_port
incorrectly") was a bit too trigger happy in removing the CPU port from
the VLAN membership because we rely on DSA to program the CPU port VLAN,
which it does, except it does not bother itself with tagged/untagged and
just usese untagged.

Having the CPU port "follow" the user ports tagged/untagged is not great
and does not allow for properly differentiating, so keep the CPU port
tagged in all VLANs.

Reported-by: Gerhard Wiesinger <lists@wiesinger.com>
Fixes: c499696e ("net: dsa: b53: Stop using dev->cpu_port incorrectly")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>