net: ethernet: improve eth_platform_get_mac_address

pci_device_to_OF_node(to_pci_dev(dev)) is the same as dev->of_node,
so we can simplify the code. In addition add an empty line before
the return statement.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>