staging: unisys: visorbus: visorchipset: Remove create_singlethread_workqueue()

With concurrency managed workqueues, use of dedicated workqueues
can be replaced by using system_wq. Drop periodic_controlvm_workqueue
by using system_wq.

Since there is only one work item periodic_controlvm_work and
different periodic_controlvm_works do not need to be ordered, increase
of concurrency level by switching to system_wq should not break anything.

cancel_delayed_work_sync() is used to ensure that work is not pending
or executing on any CPU.

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Tested-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>