media: max9286: Use the subdev active state

Use the subdev active state in the max9286 driver to store the
image format.

Replace the .open() function call with the .init_state() one and
simplify the set/get_pad_fmt() operations.

Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20240617161135.130719-11-jacopo.mondi@ideasonboard.com


Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>