gpio: merrifield: check the return value of devm_kstrdup()

devm_kstrdup() returns pointer to allocated string on success, NULL on
failure. So it is better to check the return value of it.

Before, if devm_kstrdup() fails, pinctrl_dev_name will be NULL, then the
retval below will be a negative error-code (inside gpiochip_add_pin_range(),
pinctrl_find_and_add_gpio_range()->get_pinctrl_dev_from_devname() will
finally check pinctrl_dev_name and return an error), so the failure of
devm_kstrdup() can be only implicitly caught after a long call chain.

While this patch is to explicitly catch the failure in time.
Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/gpio/gpio-merrifield.c

@@ -409,6 +409,9 @@ static int mrfld_gpio_add_pin_ranges(struct gpio_chip *chip)
 	int retval;
 
 	pinctrl_dev_name = mrfld_gpio_get_pinctrl_dev_name(priv);
+	if (!pinctrl_dev_name)
+		return -ENOMEM;
+
 	for (i = 0; i < ARRAY_SIZE(mrfld_gpio_ranges); i++) {
 		range = &mrfld_gpio_ranges[i];
 		retval = gpiochip_add_pin_range(&priv->chip, pinctrl_dev_name,