PCI: Make error code types consistent in pci_{read,write}_config_*

Callers normally treat the config space accessors as returning PCBIOS_*
error codes, not Linux error codes (or they don't look at them at all).  We
have pcibios_err_to_errno() in case the error code needs to be translated.

Fixes: 4b103883 ("PCI: Don't attempt config access to disconnected devices")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>