mlx4: replace mlx4_u64_to_mac() with u64_to_ether_addr()

mlx4_u64_to_mac() predates the common helper but doesn't
make the argument constant.

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