mp2 in mxf detected as mpeg2video

[cehoyos]

This is a regression since r14826.

$ ffmpeg -i mp2.mxf
ffmpeg version N-35550-g31c55c5, Copyright (c) 2000-2011 the FFmpeg developers
  built on Dec  8 2011 14:23:11 with gcc 4.5.3
  configuration: --cc=/usr/local/gcc-4.5.3/bin/gcc
  libavutil    51. 30. 0 / 51. 30. 0
  libavcodec   53. 41. 2 / 53. 41. 2
  libavformat  53. 24. 0 / 53. 24. 0
  libavdevice  53.  4. 0 / 53.  4. 0
  libavfilter   2. 51. 0 /  2. 51. 0
  libswscale    2.  1. 0 /  2.  1. 0
[mpeg2video @ 0x13bac40] codec type or id mismatches
    Last message repeated 39 times
[mxf @ 0x13a47a0] Could not find codec parameters (Audio: mpeg2video, 48000 Hz, 2 channels)
Input #0, mxf, from 'mp2.mxf':
  Duration: 00:00:05.00, start: 0.000000, bitrate: 4096 kb/s
    Stream #0:0: Video: mpeg2video (Main), yuv420p, 720x576 [SAR 16:15 DAR 4:3], 12000 kb/s, 25 fps, 25 tbr, 25 tbn, 50 tbc
    Stream #0:1: Audio: mpeg2video, 48000 Hz, 2 channels
At least one output file must be specified