media: nxp: imx8-isi: Drop mxc_isi_video.is_streaming field

The mxc_isi_video.is_streaming field is used to track the streaming
status of the video device. The same information is also tracked by the
videobuf2 queue. Drop the is_streaming field, and check the queue
streaming status instead.

Link: https://lore.kernel.org/r/20250813212451.22140-3-laurent.pinchart@ideasonboard.com


Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Richard Leitner <richard.leitner@linux.dev> # i.MX8MP
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>