PCI: Make pcie_set_ecrc_checking(), pcie_ecrc_get_policy() private

These interfaces:

  void pcie_set_ecrc_checking(struct pci_dev *dev);
  void pcie_ecrc_get_policy(char *str);

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/20190724233848.73327-10-skunberg.kelsey@gmail.com
Signed-off-by: Kelsey Skunberg <skunberg.kelsey@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>