staging: comedi: usbduxsigma: use comedi_cmd pointer

Use the local variable to access the comedi_cmd as a pointer instead
of getting to it from the comedi_subdevice pointer.

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>