staging: comedi: usbduxfast: remove 'intf' from the private data

The usb_interface pointer is already saved in the comedi_device
by the comedi core.

Remove the pointer from the private data and use comedi_to_usb_interface()
to get the usb_interface when needed.

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>