ACPI / hotplug / PCI: Use list_for_each_entry() for bus traversal

Replace list_for_each() + pci_bus_b() with list_for_each_entry().

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rjw@rjwysocki.net>