pci: Don't try to read cfg registers of non-existing devices

[kd]

Summary

Instead of returning 0xffs some controllers, such as Layerscape generate an external exception when someone attempts to read any register of config space of a non-existing device other than PCIR_VENDOR.
This causes a kernel panic.
Fix it by bailing during device enumeration if a device vendor register returns invalid value. (0xffff)
Use this opportunity to replace some hardcoded values with a macro.

I believe that this change won't have any unintended side-effects since it is safe to assume that if vendor == 0xffff -> hdr_type == 0xffff,
which means that we won't accidentally skip enumerating a slot.

[jhb]

Huh, interesting that the newer code (rescan) DTRT.

sys/dev/pci/pci.c

686

While this is fine, we should arguably assert that this never happens as I think we should be catching this case in all the callers? Hmm, I guess we expose this for use by CardBus, so perhaps it is best left as you have changed it. I would be happy if we could check all the callers and assert here instead though, perhaps as a followup.

4145–4147

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