The subdev .s_stream() operation shall not be called to start streaming
on an already started subdev, or stop streaming on a stopped subdev.
Remove the check that guards against that condition.

The streaming field of the driver's private structure is now unused,
drop it as well.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>