We currently have a fundamental problem in how we treat the CPU port and
its VLAN membership. As soon as a second VLAN is configured to be
untagged, the CPU automatically becomes untagged for that VLAN as well,
and yet, we don't gracefully make sure that the CPU becomes tagged in
the other VLANs it could be a member of. This results in only one VLAN
being effectively usable from the CPU's perspective.

Instead of having some pretty complex logic which tries to maintain the
CPU port's default VLAN and its untagged properties, just do something
very simple which consists in neither altering the CPU port's PVID
settings, nor its untagged settings:

- whenever a VLAN is added, the CPU is automatically a member of this
  VLAN group, as a tagged member
- PVID settings for downstream ports do not alter the CPU port's PVID
  since it now is part of all VLANs in the system

This means that a typical example where e.g: LAN ports are in VLAN1, and
WAN port is in VLAN2, now require having two VLAN interfaces for the
host to properly terminate and send traffic from/to.

Fixes: Fixes: a2482d2c ("net: dsa: b53: Plug in VLAN support")
Reported-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>