net: dsa: mv88e6xxx: do not flood CPU with unknown multicast

The DSA ports must flood unknown unicast and multicast, but the switch
must not flood the CPU ports with unknown multicast, as this results
in a lot of undesirable traffic that the network stack needs to filter
in software.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>