mlxsw: spectrum_router: Reduce scope of router struct

In a similar fashion to previous patch, the router structure
('mlxsw_sp_router') doesn't need to be accessible to anyone, but the
router code located at spectrum_router.c

Make this apparent and reduce its scope by defining 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>