arc_emac: write initial MAC address from devicetree to hw

The MAC address retrieved from dt was not actually written to the
hardware. This meant proper communication was only possible after
changing the MAC address.

Fix that by always writing the mac address during probing.

Signed-off-by: Max Schwarz <max.schwarz@online.de>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: David S. Miller <davem@davemloft.net>