lavf/mpeg: Allow users to force audio codec MLP.

Fixes part of ticket #4786.

libavformat/mpeg.c

@@ -612,7 +612,7 @@ found:
     if (st->discard >= AVDISCARD_ALL)
         goto skip;
     if (startcode >= 0xa0 && startcode <= 0xaf) {
-      if (lpcm_header_len == 6 && st->codec->codec_id == AV_CODEC_ID_MLP) {
+      if (st->codec->codec_id == AV_CODEC_ID_MLP) {
             if (len < 6)
                 goto skip;
             avio_skip(s->pb, 6);