avcodec/adpcm: Fix incorrect AVSampleFormat for sample_fmts_s16p

The AVSampleFormat list of sample_fmts_s16p is missing the trailing "P" for planar formats. AV_SAMPLE_FMT_S16 vs AV_SAMPLE_FMT_S16P

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>