mlx4: replace mlx4_mac_to_u64() with ether_addr_to_u64()

mlx4_mac_to_u64() predates and opencodes ether_addr_to_u64().
It doesn't make the argument constant so it'll be problematic
when dev->dev_addr becomes a const. Convert to the generic helper.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>