- Factor out pci_fixup_nextptr().

- Factor out pci_fixup_nextptr().
- The checking about whether 'data' register of power management capability
  exists or not is incorrect, according to PCI local bus spec:
    1) Next capability is not necessarily at a higher location than the
       current capability.
    2) Two capabilities are not necessarily consecutive.
  Set it to 0 for now (it is not used at all) and add comment about a possible
  way to test the existence of this register in power management capability.
- Clean up pci_read_extcap() a little bit.