cfg80211: Remove deprecated create_singlethread_workqueue

The workqueue "cfg80211_wq" is involved in cleanup, scan and event related
works. It queues multiple work items &rdev->event_work,
&rdev->dfs_update_channels_wk,
&wiphy_to_rdev(request->wiphy)->scan_done_wk,
&wiphy_to_rdev(wiphy)->sched_scan_results_wk, which require strict
execution ordering.
Hence, an ordered dedicated workqueue has been used.

Since it's a wireless driver, WQ_MEM_RECLAIM has been set to ensure
forward progress under memory pressure.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>