PCI: Remove pci_bus_b() and use list_for_each_entry() directly

Replace list_for_each() with list_for_each_entry(), which means we no
longer need pci_bus_b() and can remove it.

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