media: Documentation: v4l: Fix async notifier registration example

An example adding an async sub-device to a V4L2 async notifier. The name
of the variable in error handling was wrong (asd vs. my_asd).
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com> # Renesas RZ/G2L SMARC
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

Documentation/driver-api/media/v4l2-subdev.rst

@@ -234,8 +234,8 @@ These functions allocate an async sub-device descriptor which is of type struct
 						 struct my_async_subdev);
 	fwnode_handle_put(ep);
 
-	if (IS_ERR(asd))
-		return PTR_ERR(asd);
+	if (IS_ERR(my_asd))
+		return PTR_ERR(my_asd);
 
 The V4L2 core will then use these descriptors to match asynchronously
 registered subdevices to them. If a match is detected the ``.bound()``