Let the mpeg audio parser correct the codec_id.

Originally committed as revision 16582 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegaudio_parser.c

@@ -58,13 +58,16 @@ int ff_mpa_decode_header(AVCodecContext *avctx, uint32_t head, int *sample_rate,
 
     switch(s->layer) {
     case 1:
+        avctx->codec_id = CODEC_ID_MP1;
         *frame_size = 384;
         break;
     case 2:
+        avctx->codec_id = CODEC_ID_MP2;
         *frame_size = 1152;
         break;
     default:
     case 3:
+        avctx->codec_id = CODEC_ID_MP3;
         if (s->lsf)
             *frame_size = 576;
         else