net: ethernet: emac: utilize of_net's of_get_mac_address()

of_get_mac_address() reads the same "local-mac-address" property.
... But goes above and beyond by checking the MAC value properly.

printk+message seems outdated too,
so let's put dev_err in the queue.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>