mlxsw: Fail attempts to use routes with nexthop objects

Fail attempts to use nexthop objects with routes until support can be
properly added.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>