staging: comedi: usbdux: tidy up unlink and stop helpers

For aesthetic reasons, pass the comedi_device pointer to the unlink
helpers instead of the private data pointer.

All the unlink helpers simply call usb_kill_urb() to cancel any pending
transfer requests. The usb passed to usb_kill_urb() can be NULL so the
extra sanity check is not required.

The unlink helpers will always return success so just make them void
functions.

Since the stop helpers will also always return success, make them void
functions as well.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>