mlxsw: spectrum_router: Pass router to mlxsw_sp_router_schedule_work() directly

Instead of passing a notifier block and deducing the router pointer from
that in the helper, do that in the caller, and pass the result. In the
following patches, the pointer will also be made useful in the caller.

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>