gpiolib: Don't WARN on gpiod_put() for optional GPIO

In case of debug and optional GPIO requested, the gpiod_put() is not aware of
and will WARN, which is not the case.

Make gpiod_put() NULL-aware to keep silent for optional GPIOs.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

drivers/gpio/gpiolib.c

@@ -4616,7 +4616,8 @@ EXPORT_SYMBOL_GPL(gpiod_get_array_optional);
  */
 void gpiod_put(struct gpio_desc *desc)
 {
-	gpiod_free(desc);
+	if (desc)
+		gpiod_free(desc);
 }
 EXPORT_SYMBOL_GPL(gpiod_put);