dcadec: simplify an expression.
dcadec: implement request_channel_layout.
Revert "dcadec: implement request_channel_layout."

This reverts commit a9223ba25d3bd431ae4df4b3ca4498c95c9f48ca.
dcadec: implement request_channel_layout.
dcadec: wrap usage of AVCodecContext.request_channels in FF_API_REQUEST_CHANNELS.