level: fix level calculations for mutliple channels
level: fix level calculations for mutliple channels

This was broken with 7b90bf32150897a141a29a12ecab555d8c5b7fab.

GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,rawoul/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,rawoul/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,GStreamer/gst-plugins-good,sebras/gst-plugins-good,sebras/gst-plugins-good,hizukiayaka/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,sebras/gst-plugins-good,sebras/gst-plugins-good,Kurento/gst-plugins-good,GStreamer/gst-plugins-good,rawoul/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,rawoul/gst-plugins-good