staging: ath6kl: use free_netdev(netdev) instead of kfree()

It is not guaranteed that free_netdev() is macro.
Freeing netdev without free_netdev() leads to net, tx leaks.
I might lead to dereferencing freed pointer.

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