staging: comedi: das1800: use comedi_offset_munge() for analog output

The analog outputs expect 2's complement data. For aesthetics, use
the helper function to handle the munging instead of depending on
the boardinfo 'resolution'.

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>