s390: use free_netdev(netdev) instead of kfree()

Freeing netdev without free_netdev() leads to net, tx leaks.
I might lead to dereferencing freed pointer.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/

)

@@
struct net_device* dev;
@@

-kfree(dev)
+free_netdev(dev)

Signed-off-by: David S. Miller <davem@davemloft.net>