gpio: altera: change to platform_get_irq_optional to avoid false-positive error

This patch switches to platform_get_irq_optional() from
platform_get_irq() as it causes a false-positive error such as 'IRQ
index 0 not found' when IRQ is not used. The IRQ usage is optional in
this gpio-altera driver, so the error log is undesirable.
Signed-off-by: Ooi, Joyce <joyce.ooi@intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

drivers/gpio/gpio-altera.c

@@ -266,7 +266,7 @@ static int altera_gpio_probe(struct platform_device *pdev)
 	altera_gc->mmchip.gc.owner		= THIS_MODULE;
 	altera_gc->mmchip.gc.parent		= &pdev->dev;
 
-	altera_gc->mapped_irq = platform_get_irq(pdev, 0);
+	altera_gc->mapped_irq = platform_get_irq_optional(pdev, 0);
 
 	if (altera_gc->mapped_irq < 0)
 		goto skip_irq;