gpiolib: Defer on non-DT find_chip_by_name() failure

Avoid replication of error code conversion in non-DT GPIO consumers'
code by returning -EPROBE_DEFER from gpiod_find() in case a chip
identified by its label in a registered lookup table is not ready.

See https://lkml.org/lkml/2018/5/30/176 for example case.
Suggested-by: NBoris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: NJanusz Krzysztofik <jmkrzyszt@gmail.com>
Reviewed-by: NBoris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -3656,9 +3656,16 @@ static struct gpio_desc *gpiod_find(struct device *dev, const char *con_id,
 		chip = find_chip_by_name(p->chip_label);
 
 		if (!chip) {
-			dev_err(dev, "cannot find GPIO chip %s\n",
-				p->chip_label);
-			return ERR_PTR(-ENODEV);
+			/*
+			 * As the lookup table indicates a chip with
+			 * p->chip_label should exist, assume it may
+			 * still appear later and let the interested
+			 * consumer be probed again or let the Deferred
+			 * Probe infrastructure handle the error.
+			 */
+			dev_warn(dev, "cannot find GPIO chip %s, deferring\n",
+				 p->chip_label);
+			return ERR_PTR(-EPROBE_DEFER);
 		}
 
 		if (chip->ngpio <= p->chip_hwnum) {