net: lantiq_xrx200: fix use after free bug

The skb->len field is read after the packet is sent to the network
stack. In the meantime, skb can be freed. This patch fixes this bug.

Fixes: c3e6b2c3

 ("net: lantiq_xrx200: add ingress SG DMA support")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>