gpiolib: acpi: Remove never true check in acpi_get_gpiod_by_index()

The acpi_get_gpiod_by_index() never is called with adev being NULL.
Remove the redundant check.

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