[media] v4l: vsp1: Stop the pipeline upon the first STREAMOFF

The device is stopped when STREAMOFF is called on the last video node in
the pipeline. This results in possible memory corruption and/or crashes,
as userspace could free buffers while the hardware is still writing to
them, and the frame completion interrupt handler could try to access
buffers that don't exist anymore.

Fix this by stopping the pipeline upon the first STREAMOFF call, not the
last.

Reported-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>