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

This back pointer to the comedi_device is not needed in the private
data. 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>