media: i2c: ov5647: Use v4l2_async_register_subdev_sensor for lens binding

v4l2_async_register_subdev doesn't bind in lens or flash drivers,
but v4l2_async_register_subdev_sensor does.
Switch to using v4l2_async_register_subdev_sensor.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>