[media] v4l2-async: Don't fail if registered_async isn't implemented

After sub-dev registration in v4l2_async_test_notify(), the v4l2-async
core calls the registered_async callback but if a sub-dev driver does
not implement it, v4l2_subdev_call() will return a -ENOIOCTLCMD which
should not be considered an error.

Reported-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Benoit Parrot <bparrot@ti.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>