PCI/cpcihp: Use hotplug-safe pci_get_domain_bus_and_slot()

Following code has a race window between pci_find_bus() and pci_get_slot()
if PCI hotplug operation happens between them which removes the pci_bus.
So use PCI hotplug safe interface pci_get_domain_bus_and_slot() instead,
which also reduces code complexity.

    struct pci_bus *pci_bus = pci_find_bus(domain, busno);
    struct pci_dev *pci_dev = pci_get_slot(pci_bus, devfn);
Signed-off-by: NJiang Liu <jiang.liu@huawei.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>

drivers/pci/hotplug/cpcihp_generic.c

@@ -154,12 +154,8 @@ static int __init cpcihp_generic_init(void)
 	if(!r)
 		return -EBUSY;
 
-	bus = pci_find_bus(0, bridge_busnr);
-	if (!bus) {
-		err("Invalid bus number %d", bridge_busnr);
-		return -EINVAL;
-	}
-	dev = pci_get_slot(bus, PCI_DEVFN(bridge_slot, 0));
+	dev = pci_get_domain_bus_and_slot(0, bridge_busnr,
+					  PCI_DEVFN(bridge_slot, 0));
 	if(!dev || dev->hdr_type != PCI_HEADER_TYPE_BRIDGE) {
 		err("Invalid bridge device %s", bridge);
 		pci_dev_put(dev);