The decoders in this set either have a fixed channel count, or read it
from the bitstream, and thus do not require the channel count as
external information.
Fixes various regressions since
81503ac58a763a36b1f57264013b1e76acb62b68, which requires a valid channel
count for decoders which do not set this capability.
Signed-off-by: Hendrik Leppkes <h.leppkes@gmail.com>