The driver forgets to destroy workqueue in remove() similarly to what is
done when probe() fails. Add a call to destroy_workqueue() to fix it.

Since unregistration will wait for the work to finish, we do not need to
cancel/flush the work instance in remove().
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20191114023405.31477-1-hslester96@gmail.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>