media: v4l2-subdev: Make struct v4l2_subdev_stream_config private

The v4l2_subdev_stream_config structure holds configuration data for a
stream. It was meant to be used internally only, but already found its
way into the ds90ub913 driver. Now that the driver has been fixed, make
the structure private to v4l2-subdev.c to avoid using it by accident.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>