staging: comedi: adv_pci1710: tidy up analog output subdev_flags

Remove the SDF_COMMON flag, the analog reference is not programmable and
the default aref (AREF_GROUND -> SDF_GROUND) provides adequate information
about the reference.

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>