staging: comedi: addi_apci_3120: tidy up digital output subdevice init

For aesthetics, add some white space to the digital output subdevice
initialization.

Remove the subdev_flags SDF_GROUND and SDF_COMMON. These only apply to
analog subdevices. Also remove the unnecessary SDF_READABLE subdev_flag.

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>