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>