net: dsa: sja1105: deny 8021q uppers on ports

Now that best-effort VLAN filtering is gone and we are left with the
imprecise RX and imprecise TX based in VLAN-aware mode, where the tagger
just guesses the source port based on plausibility of the VLAN ID, 8021q
uppers installed on top of a standalone port, while other ports of that
switch are under a VLAN-aware bridge don't quite "just work".

In fact it could be possible to restrict the VLAN IDs used by the 8021q
uppers to not be shared with VLAN IDs used by that VLAN-aware bridge,
but then the tagger needs to be patched to search for 8021q uppers too,
not just for the "designated bridge port" which will be introduced in a
later patch.

I haven't given a possible implementation full thought, it seems maybe
possible but not worth the effort right now. The only certain thing is
that currently the tagger won't be able to figure out the source port
for these packets because they will come with the VLAN ID of the 8021q
upper and are no longer retagged to a tag_8021q sub-VLAN like the best
effort VLAN filtering code used to do. So just deny these for the
moment.

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