PCI: Use list_for_each_entry() for bus->devices traversal

Replace list_for_each() + pci_dev_b() with the simpler
list_for_each_entry().

Tested-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>