net: e1000: implement eth_write_hwaddr for DM_ETH

Implement programming MAC address to the hardware also for device model
configuration.

Fixes: b565b18a ("board: ge: bx50v3: Enable DM for PCI and ethernet")
Signed-off-by: Ian Ray <ian.ray@ge.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>