PCI: Consolidate assignment loop next round preparation

pci_assign_unassigned_root_bus_resources() and
pci_assign_unassigned_bridge_resources() have a loop that may perform
several rounds to assign resources. The code to prepare for the next round
is identical.

Consolidate the code that prepares for the next assignment round into
pci_prepare_next_assign_round().

Link: https://lore.kernel.org/r/20241216175632.4175-17-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Xiaochun Lee <lixc17@lenovo.com>