mlp: improve request_channel_layout behavior.

mlp: improve request_channel_layout behavior.

Don't decode further substreams if request_channel_layout
is a subset of the current substream's channel_layout.

Before, we would only discard further substreams if
request_channel_layout matched the substream's
channel_layout extactly, thus decoding additional
channels which the caller would probably end up downmixing.