staging: comedi: ii_pci20kc: use comedi_offset_munge()

For aesthetics, use the helper function to handle the munging of the
analog output data from offset binary to 2's complement and the analog
input data from 2's complement to offset binary.

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>