media: subdev: Support non-routing subdevs in v4l2_subdev_s_stream_helper()

At the moment v4l2_subdev_s_stream_helper() only works for subdevices
that support routing. As enable/disable_streams now also works for
subdevices without routing, improve v4l2_subdev_s_stream_helper() to do
the same.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Umang Jain <umang.jain@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>