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>
diff --git a/drivers/pinctrl/spear/pinctrl-plgpio.c b/drivers/pinctrl/spear/pinctrl-plgpio.c
index 5aebbf7..4c04505 100644
--- a/drivers/pinctrl/spear/pinctrl-plgpio.c
+++ b/drivers/pinctrl/spear/pinctrl-plgpio.c
@@ -284,7 +284,7 @@
 {
 	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);