Revert "ACPI / hotplug / PCI: Avoid doing too much for spurious notifies"

Commit 2dc41281 (ACPI / hotplug / PCI: Avoid doing too much for
spurious notifies) changed the enable_slot() to check return value of
pci_scan_slot() and if it is zero return early from the function. It
means that there were no new devices in this particular slot.

However, if a device appeared deeper in the hierarchy the code now
ignores it causing things like Thunderbolt chaining fail to recognize
new devices.

The problem with Alex Williamson's machine was solved with commit
a47d8c8e (ACPI / hotplug / PCI: Avoid parent bus rescans on spurious
device checks) and hence we should be able to restore the original
functionality that we always rescan on bus check notification.

On a device check notification we still check what acpiphp_rescan_slot()
returns and on zero bail out early.

Fixes: 2dc41281 (ACPI / hotplug / PCI: Avoid doing too much for spurious notifies)
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>