r8169: Load MAC address from device tree if present

If the system was booted using a device tree and if the device tree
contains a MAC address, use it instead of reading one from the EEPROM.
This is useful in situations where the EEPROM isn't properly programmed
or where the firmware wants to override the existing MAC address.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>