staging: comedi: usbdux: rename private data variables

The usbdux and usbduxsigma drivers are _very_ similar. For aesthetic
reasons, rename the private data variables in this driver to match
the names in the usbduxsigma driver so we can start sharing the
common code.
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>