PCI hotplug: Fix unexpected driver unregister in pciehp_acpi.c

If pcie_ports_disabled is set, pcie_port_service_register() returns
error code and select_detection_mode() should not attempt to
unregister dummy_driver and use dummy_slots.  It should return
PCIEHP_DETECT_ACPI immediately instead.

Reviewed-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Acked-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>