PCI: rename pci_host_bridge() to find_pci_root_bridge()

pci_host_bridge() looks like a C++ constructor.  Also separate
find_pci_root_bus() out.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>