Simplify enable/disable mixing

The MIXING enum isn't needed, and now returns void instead of status_t.

Change-Id: Ibe4ec24081d75ad4ab78b9c7191fc9077959c4e9
diff --git a/services/audioflinger/AudioFlinger.cpp b/services/audioflinger/AudioFlinger.cpp
index b48f23d..dcc0bd0 100644
--- a/services/audioflinger/AudioFlinger.cpp
+++ b/services/audioflinger/AudioFlinger.cpp
@@ -2197,7 +2197,7 @@
 
             // XXX: these things DON'T need to be done each time
             mAudioMixer->setBufferProvider(track);
-            mAudioMixer->enable(AudioMixer::MIXING);
+            mAudioMixer->enable();
 
             mAudioMixer->setParameter(param, AudioMixer::VOLUME0, (void *)left);
             mAudioMixer->setParameter(param, AudioMixer::VOLUME1, (void *)right);
@@ -2243,7 +2243,7 @@
                     mixerStatus = MIXER_TRACKS_ENABLED;
                 }
             }
-            mAudioMixer->disable(AudioMixer::MIXING);
+            mAudioMixer->disable();
         }
     }