dvaudio: support 12bit modes

[ami_stuff]

​http://www.datafilehost.com/d/3caa904d

​https://trac.videolan.org/vlc/ticket/458

C:\>ffmpeg -i D:\dvaudio_ntsc_12bit_32khz_stereo.wav 1\out.avi
ffmpeg version N-79107-g30d1213 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 5.3.0 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --
enable-libdcadec --enable-libfreetype --enable-libgme --enable-libgsm --enable-l
ibilbc --enable-libmodplug --enable-libmfx --enable-libmp3lame --enable-libopenc
ore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --ena
ble-librtmp --enable-libschroedinger --enable-libsnappy --enable-libsoxr --enabl
e-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-l
ibvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-li
bwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enab
le-libzimg --enable-lzma --enable-decklink --enable-zlib
  libavutil      55. 19.100 / 55. 19.100
  libavcodec     57. 28.103 / 57. 28.103
  libavformat    57. 29.100 / 57. 29.100
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 39.102 /  6. 39.102
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
[wav @ 00000000004d2400] ignoring wrong sample_count 960000
[wav @ 00000000004d2400] Estimating duration from bitrate, this may be inaccurat
e
Input #0, wav, from 'D:\dvaudio_ntsc_12bit_32khz_stereo.wav':
  Duration: 00:00:30.03, bitrate: 1726 kb/s
    Stream #0:0: Audio: dvaudio ([21][2][0][0] / 0x0215), 32000 Hz, stereo, s16,
 1726 kb/s
Output #0, avi, to '1\out.avi':
  Metadata:
    ISFT            : Lavf57.29.100
    Stream #0:0: Audio: mp3 (libmp3lame) (U[0][0][0] / 0x0055), 32000 Hz, stereo
, s16p
    Metadata:
      encoder         : Lavc57.28.103 libmp3lame
Stream mapping:
  Stream #0:0 -> #0:0 (dvaudio (native) -> mp3 (libmp3lame))
Press [q] to stop, [?] for help
size=     378kB time=00:00:30.06 bitrate= 103.0kbits/s speed=73.6x
video:0kB audio:353kB subtitle:0kB other streams:0kB global headers:0kB muxing o
verhead: 7.127193%