slcan: not call free_netdev before rtnl_unlock in slcan_open

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

This patch is a 1:1 copy of upstream slip.c commit f596c870


("slip: not call free_netdev before rtnl_unlock in slip_open").

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