staging: comedi: adl_pci6208: save raw data for analog output readback

This driver munges the raw data from the comedi core into its two's
complement form before writing it to the hardware. The last value
written to the hardware is saved in the private data for readback.

Since the comedi core always expects the data to be in offset binary
coding, make sure to save the un-munged value for readback.

Also, prefetch the current readback value before attempting to update
the hardware. This ensures that the readback value is correct if no
values are actually written.

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>