gpio: rewrite gpiochip_offset_to_desc()

gpiochip_offset_to_desc() was using gpio_to_desc(), which directly
addresses the global GPIO array we are hoping to get rid of someday.
Reimplement it using the descriptor array of the chip itself, after
checking the requested offset is within the valid bounds of the chip.
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -150,9 +150,10 @@ EXPORT_SYMBOL_GPL(gpio_to_desc);
 static struct gpio_desc *gpiochip_offset_to_desc(struct gpio_chip *chip,
 						 unsigned int offset)
 {
-	unsigned int gpio = chip->base + offset;
+	if (offset >= chip->ngpio)
+		return ERR_PTR(-EINVAL);
 
-	return gpio_to_desc(gpio);
+	return &chip->desc[offset];
 }
 
 /**