media: ov7670: use v4l2_async_unregister_subdev()

The sub-device for ov7670 is registered by v4l2_async_register_subdev().
So it should be unregistered by v4l2_async_unregister_subdev() instead of
v4l2_device_unregister_subdev().

Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>