staging: comedi: dt2811: simplify A/D reference configuration

staging: comedi: dt2811: simplify A/D reference configuration

The analog inputs are confgured with jumpers on the board to be:
  * 16 single-ended inputs
  * 8 differential inputs
  * 16 pseudo-differential inputs (common ground)

Simplify the handling of this configuration option and properly set
the subdev_flags based on the selected input mode.

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>