PCI: Rename pci_is_bridge() to pci_has_subordinate()

Previously, pci_is_bridge() returned true only when a subordinate bus
existed.  Rename pci_is_bridge() to pci_has_subordinate() to better
indicate what we're checking.

No functional change.

[bhelgaas: changelog]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>