PCI: Export pci_find_host_bridge() for use inside PCI core

The find_pci_host_bridge() function can be useful to other PCI code so
export it.  Change its name to pci_find_host_bridge().

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>