staging: wlags49_h2: 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: Vasiliy Kulikov <segooon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>