slip: not call free_netdev before rtnl_unlock in slip_open

As the description before netdev_run_todo, we cannot call free_netdev
before rtnl_unlock, fix it by reorder the code.

Signed-off-by: yangerkun <yangerkun@huawei.com>
Reviewed-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>