PCI: Move CardBus bridge scanning to setup-cardbus.c

PCI: Move CardBus bridge scanning to setup-cardbus.c

The PCI core's pci_scan_bridge_extend() contains convoluted logic specific
to setting up bus numbers for legacy CardBus bridges. Extract the CardBus
specific part out into setup-cardbus.c to make the core code cleaner and
allow omitting CardBus bridge support from modern systems.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20251219174036.16738-24-ilpo.jarvinen@linux.intel.com