media: ipu6: isys: Use v4l2_ctrl_subdev_subscribe_event()

The ipu6-isys driver uses v4l2_ctrl_subscribe_event() to handle control
event subscription on a subdev. While this works, it is the wrong API.
Use the subdev-specific v4l2_ctrl_subdev_subscribe_event() helper
instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>