mlxsw: spectrum_router: Flood unregistered multicast packets to router

Up until now we only flooded broadcast packets to the router when an L3
interface was configured on top of a bridge. However, IPv6 Neighbour
Discovery packets are trapped to the CPU inside the router and these can
be sent with a multicast address.

Flood unregistered multicast packets to the router port, so that
relevant packets could be trapped there.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>