gpio: return -ENOTSUPP if debounce cannot be set

It appears some drivers are using gpio_set_debounce()
opportunistically, i.e. without knowing whether it works or
not. (Example: input/keyboard/gpio_keys.c) to account for
this use case, return -ENOTSUPP and do not print any
warnings in this case.

Took a round over the other gpio_set_debounce() consumers
to make sure that none of them are relying on the returned
error code to be something specific.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -1763,6 +1763,9 @@ EXPORT_SYMBOL_GPL(gpio_direction_output);
  * gpio_set_debounce - sets @debounce time for a @gpio
  * @gpio: the gpio to set debounce time
  * @debounce: debounce time is microseconds
+ *
+ * returns -ENOTSUPP if the controller does not support setting
+ * debounce.
  */
 static int gpiod_set_debounce(struct gpio_desc *desc, unsigned debounce)
 {
@@ -1778,9 +1781,9 @@ static int gpiod_set_debounce(struct gpio_desc *desc, unsigned debounce)
 
 	chip = desc->chip;
 	if (!chip->set || !chip->set_debounce) {
-		pr_warn("%s: missing set() or set_debounce() operations\n",
+		pr_debug("%s: missing set() or set_debounce() operations\n",
 			__func__);
-		return -EIO;
+		return -ENOTSUPP;
 	}
 
 	spin_lock_irqsave(&gpio_lock, flags);