ffmpegdec: Set the mp3 decoder to a rank of NONE

It's just causing too many headaches. This will force people to use
a working mp3 decoder, like mad.

ext/ffmpeg/gstffmpegdec.c

@@ -2936,6 +2936,9 @@ gst_ffmpegdec_register (GstPlugin * plugin)
       case CODEC_ID_SIPR:
         rank = GST_RANK_SECONDARY;
         break;
+      case CODEC_ID_MP3:
+        rank = GST_RANK_NONE;
+        break;
       default:
         rank = GST_RANK_MARGINAL;
         break;