ACPI: power: Use str_on_off() helper function

Use the str_on_off() helper function instead of hard-coded strings.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20250209220835.78434-2-thorsten.blum@linux.dev


[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>