mlxsw: spectrum_router: mlxsw_sp_router_fini(): Extract a helper variable

Make mlxsw_sp_router_fini() more similar to the _init() function (and more
concise) by extracting the `router' handle to a named variable and using
that throughout. The availability of a dedicated `router' variable will
come in handy in following patches.

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>