PCI: Mark resources as IORESOURCE_UNSET if we can't assign them

When assigning addresses to resources, mark them with IORESOURCE_UNSET
before we start and clear IORESOURCE_UNSET if assignment is successful.
That means that if we print the resource during assignment, we will show
the size, not a meaningless address.

Also, clear IORESOURCE_UNSET if we do assign an address, so we print the
address when it is valid.

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