gpiolib: acpi: Don't contaminate return parameter in case of error

If error occurs, leave lookup parameter untouched.

There is no functional change, since all current callers just bail out
in case of error without using the assigned pieces.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>