PCI: Clean up pci_scan_child_bus_extend() loop

PCI: Clean up pci_scan_child_bus_extend() loop

pci_scan_child_bus_extend() open-codes device number iteration in the for
loop. Convert to use PCI_DEVFN() and add PCI_MAX_NR_DEVS (there seems to be
no pre-existing define for this purpose).

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Link: https://patch.msgid.link/20250610105820.7126-3-ilpo.jarvinen@linux.intel.com