When hot-adding an ACPI host bridge, use
pci_assign_unassigned_root_bus_resources() instead of
pci_assign_unassigned_bus_resources().

The former is more aggressive and will release and reassign existing
resources if necessary.  This is safe at hot-add time because no drivers
are bound to devices below the new host bridge yet.

[bhelgaas: changelog, split __init changes out for reviewability]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>