Fix segmentation fault when headset disconnects during authorization

Headset authorization callback is not being removed when the headset
disconnects during authorization. Cancel authorization function of
audio devices doesn't iterate the callbacks list properly.
diff --git a/audio/device.c b/audio/device.c
index b8ea927..76aaddd 100644
--- a/audio/device.c
+++ b/audio/device.c
@@ -781,7 +781,7 @@
 	GSList *l, *next;
 
 	for (l = priv->auths; l != NULL; l = next) {
-		struct service_auth *auth = priv->auths->data;
+		struct service_auth *auth = l->data;
 
 		next = g_slist_next(l);