IB/nes: Remove deprecated create_singlethread_workqueue

alloc_ordered_workqueue() replaces deprecated
create_singlethread_workqueue().

The workqueue "event_wq" queues work item &event->event_work and the
workqueue "disconn_wq" queues work item work (maps to
g_cm_core->disconn_wq).

WQ_MEM_RECLAIM has not been set since the workqueues are not being used
on a memory reclaim path.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>