Port 80h usually usable only on old boards with native PCI (not PCIe) buses, even if your board has PCI slot it is usually uses PCIe to PCI bridge chip. It won't get thru any of PCI Bridges, so it's unusable in PCIe system(each PCIe slot is passed via individual PCI Bridge). For debugging purposes try to use LPC Post Card and enable corresponding Coreboot option.