PCI: Unexport pci_walk_bus_locked()

PCI: Unexport pci_walk_bus_locked()

There's only one user of pci_walk_bus_locked(), and it's internal to the
PCI core.  Unexport it and make it private to drivers/pci/.

Link: https://lore.kernel.org/r/20241022224851.340648-6-kbusch@meta.com
Signed-off-by: Keith Busch <kbusch@kernel.org>
[bhelgaas: move decl to drivers/pci/pci.h]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>