vlan: Use is_vlan_dev()

Use simplified is_vlan_dev function.

Signed-off-by: Joonwoo Park <joonwpark81@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
index b529110..8141e2d 100644
--- a/net/8021q/vlan.c
+++ b/net/8021q/vlan.c
@@ -570,8 +570,7 @@
 			goto out;
 
 		err = -EINVAL;
-		if (args.cmd != ADD_VLAN_CMD &&
-		    !(dev->priv_flags & IFF_802_1Q_VLAN))
+		if (args.cmd != ADD_VLAN_CMD && !is_vlan_dev(dev))
 			goto out;
 	}