audio: in_call is a bool type

Change-Id: Ie8db6ddac7f3cb64acd4bf95be00744754a2907d
diff --git a/audio/audio_hw.h b/audio/audio_hw.h
index fec4c68..fddeffc 100644
--- a/audio/audio_hw.h
+++ b/audio/audio_hw.h
@@ -376,7 +376,7 @@
     audio_mode_t            mode;
     struct stream_in*       active_input;
     struct stream_out*      primary_output;
-    int                     in_call;
+    bool                    in_call;
     float                   voice_volume;
     bool                    mic_mute;
     bool                    bluetooth_nrec;