mlx4: constify args for const dev_addr

netdev->dev_addr will become const soon. Make sure all
functions which pass it around mark appropriate args
as const.

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