[media] v4l: vsp1: Prevent pipelines from running when not streaming

Pipelines can only be run if all their video nodes are streaming. Commit
b4dfb9b3 ("[media] v4l: vsp1: Stop the pipeline upon the first
STREAMOFF") fixed the pipeline stop sequence, but introduced a race
condition that makes it possible to run a pipeline after stopping the
stream on a video node by queuing a buffer on the other side of the
pipeline.

Fix this by clearing the buffers ready flag when stopping the stream,
which will prevent the QBUF handler from running the pipeline.

Fixes: b4dfb9b3 ("[media] v4l: vsp1: Stop the pipeline upon the first STREAMOFF")

Reported-by: Kieran Bingham <kieran@bingham.xyz>
Tested-by: Kieran Bingham <kieran@bingham.xyz>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>