rka: fix decoding of separate channels mode

[ami_stuff]

rkau.exe han an option to enable or disable separate channgels for encoded files

currently the ffmpeg's decoder is not able to decode correctly files created with this option turned off

./ffmpeg -i 16_33_s_l3_separate_stereo_off.rka -y out.wav
ffmpeg version N-109863-g4113445e9d Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
  configuration: 
  libavutil      58.  1.100 / 58.  1.100
  libavcodec     60.  2.100 / 60.  2.100
  libavformat    60.  2.100 / 60.  2.100
  libavdevice    60.  0.100 / 60.  0.100
  libavfilter     9.  1.100 /  9.  1.100
  libswscale      7.  0.100 /  7.  0.100
  libswresample   4.  9.100 /  4.  9.100
Guessed Channel Layout for Input Stream #0.0 : stereo
Input #0, rka, from '16_33_s_l3_separate_stereo_off.rka':
  Duration: 00:00:10.00, start: 0.000000, bitrate: 213 kb/s
  Stream #0:0: Audio: rka, 33333 Hz, 2 channels, s16p
Stream mapping:
  Stream #0:0 -> #0:0 (rka (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, wav, to 'out.wav':
  Metadata:
    ISFT            : Lavf60.2.100
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 33333 Hz, stereo, s16, 1066 kb/s
    Metadata:
      encoder         : Lavc60.2.100 pcm_s16le
size=    1302kB time=00:00:09.83 bitrate=1085.1kbits/s speed= 134x    
video:0kB audio:1302kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.005850%