atrac3: add CODEC_CAP_SUBFRAMES capability

the decoder can handle multiple frames in a packet

libavcodec/atrac3.c

@@ -1047,5 +1047,6 @@ AVCodec ff_atrac3_decoder =
     .init = atrac3_decode_init,
     .close = atrac3_decode_close,
     .decode = atrac3_decode_frame,
+    .capabilities = CODEC_CAP_SUBFRAMES,
     .long_name = NULL_IF_CONFIG_SMALL("Atrac 3 (Adaptive TRansform Acoustic Coding 3)"),
 };