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

BugLink: https://bugs.launchpad.net/bugs/1855313

[ Upstream commit c5510b8d ]

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: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

include/linux/gpio/consumer.h

@@ -398,7 +398,7 @@ static inline int gpiod_to_irq(const 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)