PCI: Make pci_bus_get(), pci_bus_put() private

PCI: Make pci_bus_get(), pci_bus_put() private

These interfaces:

  struct pci_bus *pci_bus_get(struct pci_bus *bus);
  void pci_bus_put(struct pci_bus *bus);

are only used in drivers/pci/ and do not need to be seen by the rest of the
kernel.  Move them to drivers/pci/pci.h so they're private to the PCI
subsystem.

Link: https://lore.kernel.org/r/20190724233848.73327-7-skunberg.kelsey@gmail.com
Signed-off-by: Kelsey Skunberg <skunberg.kelsey@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>