mlpdec: fix request_channel_layout behavior.

When request_channel_layout is 0,
all substreams should be decoded.

Signed-off-by: Tim Walker <tdskywalker@gmail.com>
mlp_parser: fix request_channel_layout behavior.

When request_channel_layout is 0,
all substreams should be decoded.

Thanks to Michael Niedermayer for spotting.

Also fix a mismatch between the parser and
decoder when request_channel_layout is a
subset of Stereo.