net: ethernet: SP7021: fix a use after free of skb->len

The netif_receive_skb() function frees "skb" so store skb->len before
it is freed.

Fixes: fd3040b9

 ("net: ethernet: Add driver for Sunplus SP7021")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YoUuy4iTjFAcSn03@kili
Signed-off-by: Jakub Kicinski <kuba@kernel.org>