gpiolib: let gpiod_request() return -EPROBE_DEFER

Patch be1a4b brought some improvements to the GPIO error handling code,
but also changed the return value of gpiod_request() when called on a
not yet initialized GPIO descriptor: it now returns -EINVAL instead of
-EPROBE_DEFER, and this affects some drivers.

This patch restores the original behavior for gpiod_request(). It is
safe to do so now that desc_to_gpio() does not rely on the GPIO
descriptor to be initialized. Other functions changed by patch be1a4b
do not see their return value affected, so these are not reverted.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reported-by: Dr. H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -1398,7 +1398,7 @@ static int gpiod_request(struct gpio_desc *desc, const char *label)
 	int			status = -EPROBE_DEFER;
 	unsigned long		flags;
 
-	if (!desc || !desc->chip) {
+	if (!desc) {
 		pr_warn("%s: invalid GPIO\n", __func__);
 		return -EINVAL;
 	}
@@ -1406,6 +1406,8 @@ static int gpiod_request(struct gpio_desc *desc, const char *label)
 	spin_lock_irqsave(&gpio_lock, flags);
 
 	chip = desc->chip;
+	if (chip == NULL)
+		goto done;
 
 	if (!try_module_get(chip->owner))
 		goto done;