ethernet: ibm/emac: use of_get_ethdev_address() to load dev_addr

A straggler I somehow missed in the automated conversion in
commit 9ca01b25

 ("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>