staging: comedi: aio_aio12_8: analog outputs are single-ended

The analog outputs are all single-ended. Remove the SDF_DIFF
subdev_flag from the subdevice initialization.

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>