media: staging: imx-csi: explicitly start media pipeline on pad 0

media: staging: imx-csi: explicitly start media pipeline on pad 0

entity->pads is an array that contains all the pads of an entity.

Calling __media_pipeline_start() or __media_pipeline_stop() on the pads,
implicitly starts the pipeline with the first pad in this array as origin.

Explicitly use the first pad to start the pipeline to make this more
obvious to the reader.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>