gpio: bail out silently on NULL descriptors

In fdeb8e1547cb9dd39d5d7223b33f3565cf86c28e
("gpio: reflect base and ngpio into gpio_device")
assumed that GPIO descriptors are either valid or error
pointers, but gpiod_get_[index_]optional() actually return
NULL descriptors and then all subsequent calls should just
bail out.

Cc: stable@vger.kernel.org
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Fixes: fdeb8e1547cb ("gpio: reflect base and ngpio into gpio_device")
Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -1367,10 +1367,13 @@ done:
 /*
  * This descriptor validation needs to be inserted verbatim into each
  * function taking a descriptor, so we need to use a preprocessor
- * macro to avoid endless duplication.
+ * macro to avoid endless duplication. If the desc is NULL it is an
+ * optional GPIO and calls should just bail out.
  */
 #define VALIDATE_DESC(desc) do { \
-	if (!desc || !desc->gdev) { \
+	if (!desc) \
+		return 0; \
+	if (!desc->gdev) { \
 		pr_warn("%s: invalid GPIO\n", __func__); \
 		return -EINVAL; \
 	} \
@@ -1381,7 +1384,9 @@ done:
 	} } while (0)
 
 #define VALIDATE_DESC_VOID(desc) do { \
-	if (!desc || !desc->gdev) { \
+	if (!desc) \
+		return; \
+	if (!desc->gdev) { \
 		pr_warn("%s: invalid GPIO\n", __func__); \
 		return; \
 	} \