int pci_cfg_space_size (struct pci_dev * dev);
Regular PCI devices have 256 bytes, but PCI-X 2 and PCI Express devices have 4096 bytes. Even if the device is capable, that doesn't mean we can access it. Maybe we don't have a way to generate extended config space accesses, or the device is behind a reverse Express bridge. So we try reading the dword at 0x100 which must either be 0 or a valid extended capability header.