Use correct type for OutputDescriptor::format

Change-Id: Ide608ef452d57da29b708180d90470361c123d1d
diff --git a/include/media/AudioSystem.h b/include/media/AudioSystem.h
index f9e625e..006af08 100644
--- a/include/media/AudioSystem.h
+++ b/include/media/AudioSystem.h
@@ -158,7 +158,7 @@
         : samplingRate(0), format(AUDIO_FORMAT_DEFAULT), channelMask(0), frameCount(0), latency(0)  {}
 
         uint32_t samplingRate;
-        int32_t format;
+        audio_format_t format;
         audio_channel_mask_t channelMask;
         size_t frameCount;
         uint32_t latency;