mlxsw: spectrum_router: Move FIB notification block to router struct

The FIB notification block logically belongs inside the router specific
struct, so move it 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>