net: rtl8169: Implement ->hwaddr_write() callback

Implement this callback that allows the MAC address to be set for the
Ethernet card. This is necessary in order for the device to be able to
receive packets for the MAC address that U-Boot advertises.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>