r6040: fix wrong assignment in r6040_free_txbufs

This patch fixes a wrong assignment in r6040_free_txbufs
on a receive skb pointer while we should actually do this
on the transmit skb pointer.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>