openflow_input: make of_flow_stats_entry.flags use enum type

The flow stats entry was still using uint16, while the flod_add etc.
messages were using enum ofp_flow_mod_flags.
diff --git a/openflow_input/standard-1.3 b/openflow_input/standard-1.3
index 19940ac..64a465d 100644
--- a/openflow_input/standard-1.3
+++ b/openflow_input/standard-1.3
@@ -1378,7 +1378,7 @@
     uint16_t priority;
     uint16_t idle_timeout;
     uint16_t hard_timeout;
-    uint16_t flags;
+    enum ofp_flow_mod_flags flags;
     pad(4);
     uint64_t cookie;
     uint64_t packet_count;