staging: comedi: me4000: only set SDF_DIFF when supported

Some of the boards supported by this driver do not have differential analog
inputs. Only set the SDF_DIFF subdev_flag when the board supports it.

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>