x86/PCI: Use pci_is_bridge() to simplify code

Use pci_is_bridge() to simplify code.  No functional change.

Requires: 326c1cda PCI: Rename pci_is_bridge() to pci_has_subordinate()
Requires: 1c86438c PCI: Add new pci_is_bridge() interface
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>