If start_streaming fails or stop_streaming is called, then all queued
buffers need to be given back to vb2.

This prevents vb2 from calling WARN_ON when it detects that this is not
done correctly.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>