Bluetooth: Fix kfree() => kfree_skb() in hci_ath.c

sk_buffs have to be freed with kfree_skb() instead of kfree().

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Gustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>