staging: comedi: usbdux: remove 'interface' from private data

This back pointer is only used for a couple dev_printk() messages and
during the detach.

For the dev_printk() we can use the dev->class_dev. In the detach we
can get the usb_interface from the comedi_device.

Do that and 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>