mlxsw: spectrum_router: Ignore IPv6 multipath notifications

IPv6 multipath notifications are about to be sent, but mlxsw is not
ready to process them, so ignore them.

The limitation will be lifted by a subsequent patch which will also stop
the kernel from sending a notification for each nexthop.

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