net: dsa: b53: Prevent tagged VLAN on port 7 for 7278

On 7278, port 7 of the switch connects to the ASP UniMAC which is not
capable of processing VLAN tagged frames. We can still allow the port to
be part of a VLAN entry, and we may want it to be untagged on egress on
that VLAN because of that limitation.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>