caif-hsi: Remove deprecated create_singlethread_workqueue

alloc_workqueue replaces deprecated create_singlethread_workqueue().

A dedicated workqueue has been used since the workitems are being used
on a packet tx/rx path. Hence, WQ_MEM_RECLAIM has been set to guarantee
forward progress under memory pressure.

An ordered workqueue has been used since workitems &cfhsi->wake_up_work
and &cfhsi->wake_down_work cannot be run concurrently.

Calls to flush_workqueue() before destroy_workqueue() have been dropped
since destroy_workqueue() itself calls drain_workqueue() which flushes
repeatedly till the workqueue becomes empty.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>