PCI: hotplug/cpqphp, fix NULL dereference

There are devices out there which are PCI Hot-plug controllers with
compaq PCI IDs, but are not bridges, hence have pdev->subordinate
NULL. But cpqphp expects the pointer to be non-NULL.

Add a check to the probe function to avoid oopses like:
BUG: unable to handle kernel NULL pointer dereference at 00000050
IP: [<f82e3c41>] cpqhpc_probe+0x951/0x1120 [cpqphp]
*pdpt = 0000000033779001 *pde = 0000000000000000
...

The device here was:
00:0b.0 PCI Hot-plug controller [0804]: Compaq Computer Corporation PCI Hotplug Controller [0e11:a0f7] (rev 11)
	Subsystem: Compaq Computer Corporation Device [0e11:a2f8]
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/pci/hotplug/cpqphp_core.c

@@ -828,7 +828,14 @@ static int cpqhpc_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 			pci_name(pdev), err);
 		return err;
 	}
+
 	bus = pdev->subordinate;
+	if (!bus) {
+		dev_notice(&pdev->dev, "the device is not a bridge, "
+				"skipping\n");
+		rc = -ENODEV;
+		goto err_disable_device;
+	}
 
 	/* Need to read VID early b/c it's used to differentiate CPQ and INTC
 	 * discovery