staging: comedi: usbduxsigma: remove 'interface' from the private data

This back pointer to the usb_interface is not used, remove it.

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>