pinctrl: SPEAr: Update error check for unsigned variables

Checking '< 0' for unsigned variables always returns false. For error
codes, use IS_ERR_VALUE() instead.
Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/spear/pinctrl-plgpio.c

@@ -284,7 +284,7 @@ static int plgpio_to_irq(struct gpio_chip *chip, unsigned offset)
 {
 	struct plgpio *plgpio = container_of(chip, struct plgpio, chip);
 
-	if (plgpio->irq_base < 0)
+	if (IS_ERR_VALUE(plgpio->irq_base))
 		return -EINVAL;
 
 	return irq_find_mapping(plgpio->irq_domain, offset);