netdev-linux: Set missing cache validity bit.

Whether a port is internal is cached to avoid requerying the kernel
every time stats are requested.  However, the cache vality bit was
never being set so the cache wasn't used.  This corrects that
oversight.

Thanks to Ben Pfaff for noticing.

lib/netdev-linux.c

@@ -654,6 +654,8 @@ netdev_linux_get_stats(const struct netdev *netdev_, struct netdev_stats *stats)
                                                      "openvswitch");
             }
         }
+
+        netdev->cache->valid |= VALID_IS_INTERNAL;
     }
 
     if (netdev->cache->is_internal) {