mlxsw: constify address in mlxsw_sp_port_dev_addr_set

Argument comes from netdev->dev_addr directly, it needs a const.

Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>