mlxsw: spectrum_router: Privatize mlxsw_sp_rif_dev()

Now that the external users of mlxsw_sp_rif_dev() have been converted in
the preceding patches, make the function static.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>