lantiq: fix broadcast packets leaking on the wrong vlan on xrx200

The ethernet driver uses a port map override via special tag to control
the ports on which multicast packets are sent. This was added to work
around an issue in the switch that was occasionally leaking packets onto
the wrong vlan.
Unfortunately the change had some leftover lines that were overwrting
the port map with a list of all ports, thus always leaking packets onto
the wront vlan.

Fix this by only enabling the override with the VLAN port map and only
if a matching VLAN port map was actually found

Signed-off-by: Felix Fietkau <nbd@nbd.name>