PCI: keystone: Use PCI_ERROR_RESPONSE to identify config read errors

Include PCI_ERROR_RESPONSE along with 0xffffffff in the comment about
identifying config read errors. This makes checks for config read errors
easier to find. Comment change only.

Link: https://lore.kernel.org/r/6ae6b071d92052dc511407513e2a7c0035aff9e7.1637243717.git.naveennaidu479@gmail.com


Signed-off-by: Naveen Naidu <naveennaidu479@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>