audio policy: use audio_devices_t when appropriate

Change-Id: Iee57cf5c3bdfefd981e1ed5958df10171f8716e6
diff --git a/include/hardware/audio_policy.h b/include/hardware/audio_policy.h
index f31bea8..43d9e83 100644
--- a/include/hardware/audio_policy.h
+++ b/include/hardware/audio_policy.h
@@ -210,7 +210,7 @@
                                         audio_stream_type_t stream);
 
     /* return the enabled output devices for the given stream type */
-    uint32_t (*get_devices_for_stream)(const struct audio_policy *pol,
+    audio_devices_t (*get_devices_for_stream)(const struct audio_policy *pol,
                                        audio_stream_type_t stream);
 
     /* Audio effect management */