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>
diff --git a/net/eth-uclass.c b/net/eth-uclass.c
index 565db15..3d0ec91 100644
--- a/net/eth-uclass.c
+++ b/net/eth-uclass.c
@@ -598,7 +598,7 @@
 		eth_env_set_enetaddr_by_index("eth", dev_seq(dev),
 					      pdata->enetaddr);
 #else
-		printf("\nError: %s address not set.\n",
+		printf("\nError: %s No valid MAC address found.\n",
 		       dev->name);
 		return -EINVAL;
 #endif