output: removed DEVICE_ON, DEVICE_OFF

To check whether a device is really on or off, we should rather check
audio_output.open, instead of managing another variable.  Wrap
audio_output.open in the inline function audio_output_is_open() and
use it instead of DEVICE_ON and DEVICE_OFF.

src/output_internal.h

@@ -109,6 +109,12 @@ struct audio_output {
  */
 extern struct notify audio_output_client_notify;
 
+static inline int
+audio_output_is_open(const struct audio_output *ao)
+{
+	return ao->open;
+}
+
 static inline int
 audio_output_command_is_finished(const struct audio_output *ao)
 {