net: dsa: sja1105: hide all multicast addresses from "bridge fdb show"

Commit 4d942354 ("net: dsa: sja1105: offload bridge port flags to
device") has partially hidden some multicast entries from showing up in
the "bridge fdb show" output, but it wasn't enough. Addresses which are
added through "bridge mdb add" still show up. Hide them all.

Fixes: 291d1e72 ("net: dsa: sja1105: Add support for FDB and MDB management")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>