staging: comedi: usbduxsigma: rename the private data struct

For aesthetic reasons, rename the struct used for the comedi_device
private data from usbduxsub to usbduxsigma_private.
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>