ofp-util: Rename OFPUTIL_INVALID to OFPUTIL_MSG_INVALID.

An upcoming commit will introduce new OPFUTIL_* constants for actions.  It
seems best to be able to visually distinguish the contants.  Most of the
existing constants start with a good prefix, but OFPUTIL_INVALID does not,
so rename it.

lib/ofp-util.c

@@ -725,8 +725,8 @@ ofputil_decode_msg_type(const struct ofp_header *oh,
     }
     if (error) {
         static const struct ofputil_msg_type ofputil_invalid_type = {
-            OFPUTIL_INVALID,
-            0, "OFPUTIL_INVALID",
+            OFPUTIL_MSG_INVALID,
+            0, "OFPUTIL_MSG_INVALID",
             0, 0
         };