staging: comedi: adl_pci6208: use comedi_offset_munge()

Use comedi_offset_munge() to munge the offset binary values from the
comedi core into the two's complement form that the hardware expects.

This removes the hard coded assumption that the analog outputs are
16-bits and instead does the munging based on the s->maxdata.

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>