staging: comedi: ii_pci20kc: save unmunged data for ao readback

The unmunged data should be saved for readback not the munged data.

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>