usb: ftdi-elan: Remove deprecated create_singlethread_workqueue

usb: ftdi-elan: Remove deprecated create_singlethread_workqueue

The status workqueue is involved in initializing the Uxxx and polling
the Uxxx until a supported PCMCIA CardBus device is detected.
It then starts the command and respond workqueues and then loads the
module that handles the device, after which it just polls the Uxxx
looking for card ejects.

The command and respond workqueues are involved in implementing a command
sequencer for communicating with the firmware on the other side of
the FTDI chip in the Uxxx.

These workqueues have only a single work item each and hence they do not
require ordering. Also, none of the above workqueues are being used on a
memory recliam path. Hence, the singlethreaded workqueues have been
replaced with the use of system_wq.

System workqueues have been able to handle high level of concurrency
for a long time now and hence it's not required to have a singlethreaded
workqueue just to gain concurrency. Unlike a dedicated per-cpu workqueue
created with create_singlethread_workqueue(), system_wq allows multiple
work items to overlap executions even on the same CPU; however, a
per-cpu workqueue doesn't have any CPU locality or global ordering
guarantee unless the target CPU is explicitly specified and thus the
increase of local concurrency shouldn't make any difference.

The work items have been sync cancelled because they are self-requeueing
and need to wait for the in-flight work item to finish before proceeding
with destruction. Hence, they have been sync cancelled in
ftdi_status_cancel_work(), ftdi_command_cancel_work() and
ftdi_response_cancel_work(). These functions are called in
ftdi_elan_exit() to ensure that there are no pending work items while
disconnecting the driver.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>