PCI: Use res->parent to check if resource is assigned

reassign_resources_sorted() uses resource_size() to select between
pci_assign_resource() and pci_reassign_resource(). Due to twisted way
bridge window sizing in pbus_size_mem() sets resource sizes to 0, it works
to match into IOV resources but that is going to be changed by an upcoming
change.

Replace resource_size() check with res->parent check that is the true
dividing line in between whether assign or reassign function should be used
for the resource.

Link: https://lore.kernel.org/r/20241216175632.4175-24-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>