net: eth-uclass: Improve error message when MAC is not found

While bringinp up a new board without the MAC fuses programmed,
the following error message was observed:

Error: ethernet@30bf0000 address not set.

Improve the error message to make it clearer the reason of
the failure.

Signed-off-by: Fabio Estevam <festevam@denx.de>