A config read from a PCI device that doesn't exist or doesn't respond
causes a PCI error. There's no real data to return to satisfy the CPU read,
so most hardware fabricates ~0 data.

Use PCI_SET_ERROR_RESPONSE() to set the error response when we think the
device has already been disconnected.

This helps unify PCI error response checking and make error checks
consistent and easier to find.

Link: https://lore.kernel.org/r/29db0a6874716db80757e4e3cdd03562f13eb0cb.1637243717.git.naveennaidu479@gmail.comSigned-off-by: Naveen Naidu <naveennaidu479@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>