net: Rename eth.c to eth_lecacy.c

Rename this file to make it clear it is for the old networking drivers
and not for use with driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>