mlxsw: spectrum_router: Use router.lb_crif instead of .lb_rif_index

mlxsw: spectrum_router: Use router.lb_crif instead of .lb_rif_index

A previous patch added a pointer to loopback CRIF to the router data
structure. That makes the loopback RIF index redundant, as everything
necessary can be derived from the CRIF. Drop the field and adjust the code
accordingly.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Link: https://lore.kernel.org/r/8637bf959bc5b6c9d5184b9bd8a0cd53c5132835.1687438411.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>