ACPI / Wakeup: Simplify enabling of wakeup devices

To simplify the enabling of wakeup devices during system suspend and
hibernation, merge acpi_enable_wakeup_device_prep() with
acpi_disable_wakeup_device() and remove unnecessary (and no longer
valid) comments from the latter.  Rename acpi_enable_wakeup_device()
to acpi_enable_wakeup_devices() and acpi_disable_wakeup_device()
to acpi_disable_wakeup_devices(), because these functions usually
operate on multiple device objects.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>