x86/PCI: Don't track firmware-assigned BAR values for hot-added devices

The BIOS doesn't assign BAR values for hot-added devices, so don't
bother saving the original values when we enumerate these devices.

[bhelgaas: changelog, return constant 0 in pcibios_retrieve_fw_addr]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>