chmap: fix oddity due to ambiguous 6.1 ffmpeg channel layout

FFmpeg (as well as Libav) have two layouts called "6.1":
AV_CH_LAYOUT_6POINT1 and AV_CH_LAYOUT_6POINT1_BACK. We call them "6.1"
and "6.1(back)". Change the default layout for 7 channels as well to
return the same layout as av_get_default_channel_layout(). (Looks a bit
questionable, but for now it's better to follow FFmpeg.)