Introduce helper functions pci_bus_{get|put}() to manage PCI bus
reference count.
Signed-off-by: NJiang Liu <jiang.liu@huawei.com>
Signed-off-by: NYijing Wang <wangyijing@huawei.com>
Signed-off-by: NGu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>