mmc: update workqueue usages

Workqueue creation API has been updated and flush_scheduled_work() is
deprecated and scheduled to be removed.

* core/core.c: Use alloc_ordered_workqueue() instead of
  create_singlethread_workqueue().  This removes an unnecessary
  rescuer.

* host/omap.c: Create, use and flush mmc_omap_wq instead of the
  system_wq.

* Flush host->mmc_carddetect_work directly on removal instead of using
  flush_scheduled_work().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Chris Ball <cjb@laptop.org>
Cc: linux-mmc@vger.kernel.org