media: imx: imx7_mipi_csis: Power off the source when stopping streaming

The .s_stream() implementation incorrectly powers on the source when
stopping the stream. Power it off instead.

Fixes: 7807063b ("media: staging/imx7: add MIPI CSI-2 receiver subdev for i.MX7")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>