PCI: Separate out pci_assign_unassigned_bus_resources()

It is main portion of pci_rescan_bus().

Separate it out and prepare to use it for PCI root bus hot add later.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>