staging: comedi: rti800: use comedi_offset_munge()

Use the comedi_offset_munge() helper to do the two's complement
to comedi offset binary and comedi offset binary to two's complement
conversions.

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>