gpiolib: use gpiochip_get_desc() where applicable

Whenever retrieving a descriptor from a gpiochip: use the provided
helper which checks for errors.
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20200219094702.6463-1-brgl@bgdev.plSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -3143,7 +3143,9 @@ const char *gpiochip_is_requested(struct gpio_chip *chip, unsigned offset)
 	if (offset >= chip->ngpio)
 		return NULL;
 
-	desc = &chip->gpiodev->descs[offset];
+	desc = gpiochip_get_desc(chip, offset);
+	if (IS_ERR(desc))
+		return NULL;
 
 	if (test_bit(FLAG_REQUESTED, &desc->flags) == 0)
 		return NULL;