PCI: Don't try to claim IORESOURCE_UNSET resources

If the IORESOURCE_UNSET bit is set, it means we haven't assigned an address
yet, so don't try to claim the region.

Also, make the error messages more uniform and add info about which BAR is
involved.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>