PCI: Add new pci_is_bridge() interface

Add a helper function to check a device's header type for PCI bridge or
CardBus bridge.

Requires: 326c1cda

 PCI: Rename pci_is_bridge() to pci_has_subordinate()
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>