net: Don't call board/cpu_eth_init() with driver model

We should avoid weak functions with driver model. Existing boards that use
driver model don't need them, so let's kill them off.

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