audio: refuse to change playback speed with spdif

Handle the failure gracefully, instead of exploding and disabling audio.
Just set the speed back to 1.0.

Also remove the AF_DETACH from af_scaletempo. This actually created a
dangling pointer in af_add(), a tricky consequence of af_add()
reconfiguring the filter chain and the newly added filter using
AF_DETACH. Fortunately the AF_DETACH is not needed (and probably never
worked - it comes from MPlayer times, and MPlayer also disables audio
when trying to change speed with spdif).

audio/filter/af_scaletempo.c

@@ -305,7 +305,7 @@ static int control(struct af_instance *af, int cmd, void *arg)
 
         if (AF_FORMAT_IS_SPECIAL(data->format)) {
             MP_ERR(af, "Changing speed is not supported with spdif formats.\n");
-            return AF_DETACH;
+            return AF_ERROR;
         }
 
         mp_audio_force_interleaved_format(data);