PCI: Use PCI_EXP_SLTCAP_PSN mask when extracting slot number

Use PCI_EXP_SLTCAP_PSN mask to make it easier to find where the
Physical Slot Number is used.

The Physical Slot Number is bits 31:19 of the Slot Capabilities Register,
and slot_cap is a u32, so the mask is technically unnecessary, but it's
helpful for human readers.
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>

drivers/pci/hotplug/pciehp_acpi.c

@@ -90,7 +90,7 @@ static int __init dummy_probe(struct pcie_device *dev)
 	slot = kzalloc(sizeof(*slot), GFP_KERNEL);
 	if (!slot)
 		return -ENOMEM;
-	slot->number = slot_cap >> 19;
+	slot->number = (slot_cap & PCI_EXP_SLTCAP_PSN) >> 19;
 	list_for_each_entry(tmp, &dummy_slots, list) {
 		if (tmp->number == slot->number)
 			dup_slot_id++;