ethernet: ibmveth: use ether_addr_to_u64()

We'll want to make netdev->dev_addr const, remove the local
helper which is missing a const qualifier on the argument
and use ether_addr_to_u64().

Similar story to mlx4.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>