[media] dt3155v4l: correctly start and stop streaming

Don't start streaming when a buffer is queued, instead implement the
start_streaming op and do it there, leaving it up to the vb2 framework
to call start_streaming when enough buffers have been queued.

And don't stop streaming from within the interrupt routine, instead do
that in stop_streaming.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>