ACPI / hotplug / PCI: Redefine enable_device() and disable_device()

Notice that functions enable_device() and disable_device() cannot
fail and their return values are ignored in the majority of places,
so redefine them as void and use the opportunity to change their
names to enable_slot() and disable_slot(), respectively, which much
better reflects what they do.

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