net: constify netif_is_* helpers net_device param

As suggested by Eric, these helpers should have const dev param.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/if_vlan.h b/include/linux/if_vlan.h
index 67ce5bd..05f5879 100644
--- a/include/linux/if_vlan.h
+++ b/include/linux/if_vlan.h
@@ -73,7 +73,7 @@
 /* found in socket.c */
 extern void vlan_ioctl_set(int (*hook)(struct net *, void __user *));
 
-static inline bool is_vlan_dev(struct net_device *dev)
+static inline bool is_vlan_dev(const struct net_device *dev)
 {
         return dev->priv_flags & IFF_802_1Q_VLAN;
 }