staging: comedi: usbdux: remove usb_device back pointer from private data

The usb_device can be found when needed using the comedi_to_usb_dev()
helper. Use that instead the remove the back pointer from the private
data.

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