avfilter/dnn: fix missing set default value to options

There are two options which use non-zero default value: async and
batch_size of openvino. init_model_ov checks and set batch_size to
one when batch_size is equal to zero, so the only option affected
by missing default value is async. Now async works as expected.

libavfilter/dnn/dnn_interface.c

@@ -98,6 +98,8 @@ void ff_dnn_init_child_class(DnnContext *ctx)
     for (int i = 0; i < FF_ARRAY_ELEMS(dnn_backend_info_list); i++) {
         const AVClass **ptr = (const AVClass **) ((char *) ctx + dnn_backend_info_list[i].offset);
         *ptr = dnn_backend_info_list[i].class;
+        // Set default values after the class pointer is set
+        av_opt_set_defaults(ptr);
     }
 }