gpiolib: Fix return value of gpio_to_desc() stub if !GPIOLIB

mainline inclusion
from mainline-5.0-rc1
commit c5510b8d
category: bugfix
bugzilla: 5539
CVE: NA

---------------------------

If CONFIG_GPOILIB is not set, the stub of gpio_to_desc() should return
the same type of error as regular version: NULL.  All the callers
compare the return value of gpio_to_desc() against NULL, so returned
ERR_PTR would be treated as non-error case leading to dereferencing of
error value.

Fixes: 79a9becd ("gpiolib: export descriptor-based GPIO interface")
Signed-off-by: NKrzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NJason Yan <yanaijie@huawei.com>
Reviewed-by: Nzhengbin <zhengbin13@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

include/linux/gpio/consumer.h

@@ -475,7 +475,7 @@ static inline int gpiod_set_consumer_name(struct gpio_desc *desc,
 
 static inline struct gpio_desc *gpio_to_desc(unsigned gpio)
 {
-	return ERR_PTR(-EINVAL);
+	return NULL;
 }
 
 static inline int desc_to_gpio(const struct gpio_desc *desc)