staging: otus: 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: NVasiliy Kulikov <segooon@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/otus/usbdrv.c

@@ -690,7 +690,7 @@ void usbdrv_clear_structs(struct net_device *dev)
 
     kfree(macp);
 
-    kfree(dev);
+    free_netdev(dev);
 
 }