staging: comedi: usbdux: use comedi_offset_munge()

Use the comedi_offset_munge() helper to convert the hardware two's
complement values to the offset binary format expected by comedi.

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>