microblaze: Correct build error in eth-uclass.c

This fixes the following error when building microblaze-generic:

net/eth-uclass.c: In function 'eth_post_probe':
net/eth-uclass.c:466:18: error: 'gd' undeclared (first use in this function)
    ops->start += gd->reloc_off;

Fixes: db9391e1 ("net: Move driver-model code into its own file")

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>