PCI/AER: Don't read upstream ports below fatal errors

The AER driver has never read the config space of an endpoint that reported
a fatal error because the link to that device is considered unreliable.

An ERR_FATAL from an upstream port almost certainly indicates an error on
its upstream link, so we can't expect to reliably read its config space for
the same reason.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Sinan Kaya <okaya@kernel.org>