FP4: Don't try to play many-channel audio as is

This device doesn't have surround sound, as nice as that would be, so
stop pretending. Otherwise, we get weird behavior, like playing surround
sound audio at full volume regardless of the media volume or mute state.
Now, it plays properly.

Issue: calyxos#2075
Change-Id: Ic530906a9dacb2699e32f2ed91f38fab0dad1492