lu_fsm: result cb: use proper cause type (warning/coverity)

enum gsm48_gmm_cause is the wrong enum to pass to lu_fsm_failure(). Use enum
gsm48_reject_value instead.

Change-Id: If661f72056decb28c0ee82ad2449630a24d4f31c
diff --git a/src/libvlr/vlr_lu_fsm.c b/src/libvlr/vlr_lu_fsm.c
index bf23551..717db42 100644
--- a/src/libvlr/vlr_lu_fsm.c
+++ b/src/libvlr/vlr_lu_fsm.c
@@ -1188,8 +1188,8 @@
 			/* continue in MSC ?!? */
 		} else {
 			/* unsuccessful case */
-			enum gsm48_gmm_cause cause =
-				*(enum gsm48_gmm_cause *)data;
+			enum gsm48_reject_value cause =
+				*(enum gsm48_reject_value *)data;
 			/* Ignoring standalone mode for now. */
 			if (0 /* procedure_error && vlr->cfg.standalone_mode */) {
 				osmo_fsm_inst_state_chg(fi,