libavutil/channel_layout: Correctly return layout when channel specification ends with a trailing 'c'.

Return layout when FF_API_GET_CHANNEL_LAYOUT_COMPAT is set even if the
layout itself is not in the deprecated style.
Signed-off-by: Simon Thelen <ffmpeg-dev@c-14.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>