ethernet: ibm/emac: use of_get_ethdev_address() to load dev_addr
A straggler I somehow missed in the automated conversion in
commit 9ca01b25dfff ("ethernet: use of_get_ethdev_address()").
Use the new helper instead of using netdev->dev_addr directly.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>