platform/chrome: cros_ec_lpc: Use platform_get_irq_optional() for optional IRQs

As platform_get_irq() now prints an error when the interrupt does not
exist, use platform_get_irq_optional() to get the IRQ which is optional
to avoid below error message during probe:

  [    5.113502] cros_ec_lpcs GOOG0004:00: IRQ index 0 not found
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>

drivers/platform/chrome/cros_ec_lpc.c

@@ -396,7 +396,7 @@ static int cros_ec_lpc_probe(struct platform_device *pdev)
 	 * Some boards do not have an IRQ allotted for cros_ec_lpc,
 	 * which makes ENXIO an expected (and safe) scenario.
 	 */
-	irq = platform_get_irq(pdev, 0);
+	irq = platform_get_irq_optional(pdev, 0);
 	if (irq > 0)
 		ec_dev->irq = irq;
 	else if (irq != -ENXIO) {