gpiolib: acpi: Simplify error handling in __acpi_find_gpio()

Now that we don't perform anything on the GPIO descriptor,
we may simplify the error path in newly introduced helper.
Do it so.

Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>