spdif: Allow muxing Pure Lossless DTS-HD

[Carl Eugen Hoyos]

​http://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket4619/
The sample from ticket #4619 cannot be muxed into spdif:

$ ffmpeg -i Pure_Lossless_DTS-HD~stereo-sample.mka -acodec copy -dtshd_rate 768k out.spdif
ffmpeg version N-93098-g5b32f94 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 6.4.0 (GCC)
  configuration: --enable-gpl
  libavutil      56. 26.100 / 56. 26.100
  libavcodec     58. 46.100 / 58. 46.100
  libavformat    58. 26.100 / 58. 26.100
  libavdevice    58.  6.101 / 58.  6.101
  libavfilter     7. 48.100 /  7. 48.100
  libswscale      5.  4.100 /  5.  4.100
  libswresample   3.  4.100 /  3.  4.100
  libpostproc    55.  4.100 / 55.  4.100
Input #0, matroska,webm, from 'Pure_Lossless_DTS-HD~stereo-sample.mka':
  Metadata:
    encoder         : libebml v1.3.1 + libmatroska v1.4.2
    creation_time   : 2015-06-12T17:35:51.000000Z
  Duration: 00:05:17.61, start: 0.000000, bitrate: 948 kb/s
    Stream #0:0(por): Audio: dts (DTS-HD MA), 48000 Hz, stereo, s16p (default)
Output #0, spdif, to 'out.spdif':
  Metadata:
    encoder         : Lavf58.26.100
    Stream #0:0(por): Audio: dts (DTS-HD MA), 48000 Hz, stereo, s16p (default)
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
[spdif @ 0x3400d00] stray DTS-HD frame
av_interleaved_write_frame(): Invalid data found when processing input
size=       0kB time=00:00:00.00 bitrate=N/A speed=   0x
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Conversion failed!

[Balling]

But... Optical in ffmpeg is maximum 768k. I suppose it can do more in some real applications (that is PCM 48kHz/16bit or DTS core that both are 1536 kbit/s). Also, again, this is NOT DTS-HD. That is DTS-HD MA.