ACPI / hotplug / PCI: Do not check SLOT_ENABLED in enable_device()

With Thunderbolt you can daisy chain devices: connect new devices to
an already plugged one.  In that case the "hotplug slot" is already
enabled, but we still want to look for new PCI devices behind it.

Reuse enable_device() to scan for new PCI devices on enabled slots
and push the SLOT_ENABLED check up into acpiphp_enable_slot().

[rjw: Rebased, modified the changelog]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/pci/hotplug/acpiphp_glue.c

@@ -545,9 +545,6 @@ static int __ref enable_device(struct acpiphp_slot *slot)
 	int num, max, pass;
 	LIST_HEAD(add_list);
 
-	if (slot->flags & SLOT_ENABLED)
-		goto err_exit;
-
 	list_for_each_entry(func, &slot->funcs, sibling)
 		acpiphp_bus_add(func_to_handle(func));
 
@@ -1024,11 +1021,14 @@ void acpiphp_remove_slots(struct pci_bus *bus)
  */
 int acpiphp_enable_slot(struct acpiphp_slot *slot)
 {
-	int retval;
+	int retval = 0;
 
 	mutex_lock(&slot->crit_sect);
+
 	/* configure all functions */
-	retval = enable_device(slot);
+	if (!(slot->flags & SLOT_ENABLED))
+		retval = enable_device(slot);
+
 	mutex_unlock(&slot->crit_sect);
 	return retval;
 }