staging: comedi: pcl711: add differential input support for 8112 boards

The ACL-8112 boards can do differential analog inputs. Add support for
this 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>