audio policy: use audio_devices_t when appropriate

Change-Id: Iee57cf5c3bdfefd981e1ed5958df10171f8716e6
diff --git a/modules/audio/audio_policy.c b/modules/audio/audio_policy.c
index 5fb6095..8d7e5c3 100644
--- a/modules/audio/audio_policy.c
+++ b/modules/audio/audio_policy.c
@@ -186,7 +186,7 @@
     return 0;
 }
 
-static uint32_t ap_get_devices_for_stream(const struct audio_policy *pol,
+static audio_devices_t ap_get_devices_for_stream(const struct audio_policy *pol,
                                           audio_stream_type_t stream)
 {
     return 0;