ACPI: Always check if _PRW is present before trying to evaluate it

Before evaluating _PRW for devices that are reported as inactive or
not present by their _STA control methods we should check if those
methods are actually present (otherwise the evaulation of _PRW will
obviously fail and a scary message will be printed unnecessarily).

Reported-by: Andreas Mohr <andi@lisas.de>
Reported-by: Maciej Rutecki <maciej.rutecki@gmail.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>