auxdisplay: ht16k33: remove private workqueue

There is no need for the driver to use private workqueue, standard system
workqueue should suffice as they are going to use the same worker pool
anyway.

Acked-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>