ACPI: PM: Fix device wakeup power reference counting error

Fix a device wakeup power reference counting error introduced by
commit a2d7b2e0 ("ACPI: PM: Fix sharing of wakeup power
resources") because of a coding mistake.

Fixes: a2d7b2e0

 ("ACPI: PM: Fix sharing of wakeup power resources")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>