net/mlx5: Constify mac address pointer

Since none of the functions need to modify the input mac address,
constify them.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>