staging: comedi: adq12b: remove 'differential' from private data

This member of the private data is only used during the attach to
determine if there are 8 differential or 16 single-ended analog
inputs. Remove the member and use the comedi_devconfig option
directly.

Also, fix the subdev_flags when differential analog inputs are used.

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>