MIPS: PCI: Return PCIBIOS_* from tx4927_pci_config_read/write()

pci_ops .read/.write must return PCIBIOS_* codes but
tx4927_pci_config_read/write() return -1 when mkaddr() cannot find
devfn from the root bus. Return PCIBIOS_DEVICE_NOT_FOUND instead and
pass that onward in the call chain instead of overwriting the return
value.

Also converts 0 -> PCIBIOS_SUCCESSFUL which has only cosmetic impact.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmal.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>