ACPI / hotplug / PCI: Simplify register_slot()

The err label in register_slot() is only jumped to from one place,
so move the code under the label to that place and drop the label.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>