PCI: Make pci_save_vc_state(), pci_restore_vc_state(), etc private

PCI: Make pci_save_vc_state(), pci_restore_vc_state(), etc private

These Virtual Channel interfaces:

  int pci_save_vc_state(struct pci_dev *dev);
  void pci_restore_vc_state(struct pci_dev *dev);
  void pci_allocate_vc_save_buffers(struct pci_dev *dev);

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/20190724233848.73327-5-skunberg.kelsey@gmail.com
Signed-off-by: Kelsey Skunberg <skunberg.kelsey@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>