ao_alsa: fix return value for volume operations with spdif

This AO pretended to support volume operations when in spdif passthrough
mode, but actually did nothing. This is wrong: at least the GET
operations must write their argument. Signal that volume is unsupported
instead.

This was probably a hack to prevent insertion of volume filters or so,
but it didn't work anyway, while recovering after failed volume filter
insertion does work, so this is not needed at all.

audio/out/ao_alsa.c

@@ -101,7 +101,7 @@ static int control(struct ao *ao, enum aocontrol cmd, void *arg)
         float f_multi;
 
         if (AF_FORMAT_IS_IEC61937(ao->format))
-            return CONTROL_TRUE;
+            return CONTROL_FALSE;
 
         //allocate simple id
         snd_mixer_selem_id_alloca(&sid);