mlxsw: spectrum_router: Don't ignore IPv6 notifications

We now have all the necessary IPv6 infrastructure in place, so stop
ignoring these notifications.

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>