Use preserve_config in place of checking for PCI_PROBE_ONLY flag to enable
support for "linux,pci-probe-only" on a per host bridge basis.

This also obviates the use of adding PCI_REASSIGN_ALL_BUS flag if
!PCI_PROBE_ONLY, as pci_assign_unassigned_root_bus_resources() takes care
of reassigning the resources that are not already claimed.

Link: https://lore.kernel.org/r/20240508174138.3630283-5-vidyas@nvidia.comSigned-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>