bpa10x: free sk_buff with kfree_skb

Inspired by Sergio Luis' similar patches, I finally found
a case which is trivial enough that spatch won't choke
on it.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>