audio: Directly link voice data <-> session

This is part of a cleanup series.
In the future, we should reconsider the struct usage and (maybe?) move
the members into one common place.

Change-Id: I3218d75acb0c4965332ba0de30b40ca7821596a7
diff --git a/audio/audio_hw.h b/audio/audio_hw.h
index 4677f29..820d355 100644
--- a/audio/audio_hw.h
+++ b/audio/audio_hw.h
@@ -372,7 +372,7 @@
     float volume;
     bool  bluetooth_nrec;
     bool  bluetooth_wb;
-    void  *session;
+    struct voice_session *session;
 };
 
 struct audio_device {