batman-adv: checkpatch - comparison to NULL could be rewritten

Signed-off-by: NMarek Lindner <mareklindner@neomailbox.ch>

net/batman-adv/soft-interface.c

@@ -818,7 +818,7 @@ static int batadv_softif_slave_add(struct net_device *dev,
 	int ret = -EINVAL;
 
 	hard_iface = batadv_hardif_get_by_netdev(slave_dev);
-	if (!hard_iface || hard_iface->soft_iface != NULL)
+	if (!hard_iface || hard_iface->soft_iface)
 		goto out;
 
 	ret = batadv_hardif_enable_interface(hard_iface, dev->name);