tun: Remove a dead line of code

Remove an unnecessary assignment.

Signed-off-by: Paul Moore <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/tun.c

···
 				goto err_free_sk;
 		}
 
-		err = -EINVAL;
 		err = register_netdevice(tun->dev);
 		if (err < 0)
 			goto err_free_sk;