Document AudioFlinger::nextUniqueId()

Change-Id: Iafe96f1c10bd85cb23a2553945ca68aa601dc2eb
diff --git a/services/audioflinger/AudioFlinger.cpp b/services/audioflinger/AudioFlinger.cpp
index 50179c5..c558568 100644
--- a/services/audioflinger/AudioFlinger.cpp
+++ b/services/audioflinger/AudioFlinger.cpp
@@ -2089,7 +2089,7 @@
 
 uint32_t AudioFlinger::nextUniqueId()
 {
-    return android_atomic_inc(&mNextUniqueId);
+    return (uint32_t) android_atomic_inc(&mNextUniqueId);
 }
 
 AudioFlinger::PlaybackThread *AudioFlinger::primaryPlaybackThread_l() const