I use Odroid C2, I had same issue in most of the systems DietPi, Librelec and CoreElec.The only kernel avaiable for Odroid at the moment is 3.14.x or 3.16.x and both had this bug. The only the workaround for me was to use “Pulseaudio (Bluetooth)” as the “Audio Device” in Kodi settings, ignore that it says bluetooth,. Then run (addded this script in autostart.sh) following script, so that Pulseaudio detects USB device:
pactl load-module module-udev-detect
I have feeling that this bug is due to snd-usb-audio kernel module reporting the channles as FL and UKNOWN to Kodi when ALSA is being used. May be that is fixed in later version of module.