net: dsa: lantiq: Allow special tags only on CPU port

Allow the special tag in ingress only on the CPU port and not on all
ports. A packet with a special tag could circumvent the hardware
forwarding and should only be allowed on the CPU port where Linux
controls the port.

Fixes: 14fceff4 ("net: dsa: Add Lantiq / Intel DSA driver for vrx200)"
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>