gpio: sysfs: correct error handling on 'value' attribute read.

'value' attribute is supposed to only return 0 or 1 according to
the documentation.
With today's implementation, if gpiod_get_value_cansleep() fails
the printed 'value' is a negative value.

This patch ensures that an error is returned on read instead.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib-sysfs.c

@@ -106,8 +106,12 @@ static ssize_t value_show(struct device *dev,
 
 	mutex_lock(&data->mutex);
 
-	status = sprintf(buf, "%d\n", gpiod_get_value_cansleep(desc));
+	status = gpiod_get_value_cansleep(desc);
+	if (status < 0)
+		goto err;
 
+	status = sprintf(buf, "%d\n", status);
+err:
 	mutex_unlock(&data->mutex);
 
 	return status;