clocksource/drivers/keystone: Fix bad NO_IRQ usage

The current code assumes the 'irq_of_parse_and_map' will return NO_IRQ in case
of failure. Unfortunately, the NO_IRQ is not consistent across the different
architectures and we must not rely on it.

NO_IRQ is equal to '-1' on ARM and 'irq_of_parse_and_map' returns '0' in case
of an error. Hence, the latter won't be detected and will lead to a crash.

Fix this by just checking 'irq' is different from zero.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/timer-keystone.c

@@ -152,7 +152,7 @@ static void __init keystone_timer_init(struct device_node *np)
 	int irq, error;
 
 	irq  = irq_of_parse_and_map(np, 0);
-	if (irq == NO_IRQ) {
+	if (!irq) {
 		pr_err("%s: failed to map interrupts\n", __func__);
 		return;
 	}