control_if: Don't use magic number '5' when allocating vector

We have a proper constant for this (_LAST_CTRL_NODE), so let's use it.

Change-Id: I46275e644166156cb665da70d2964008f1c6cd88
diff --git a/src/ctrl/control_if.c b/src/ctrl/control_if.c
index 6ab34c7..c4b6f91 100644
--- a/src/ctrl/control_if.c
+++ b/src/ctrl/control_if.c
@@ -706,7 +706,7 @@
 	if (ctrl_initialized)
 		return 0;
 
-	ctrl_node_vec = vector_init(5);
+	ctrl_node_vec = vector_init(_LAST_CTRL_NODE);
 	if (!ctrl_node_vec)
 		goto err;