nios2: fix error handling of irq_of_parse_and_map

Return value of irq_of_parse_and_map() is unsigned int, with 0
indicating failure, so testing for negative result never works.
Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Reviewed-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <lftan@altera.com>

arch/nios2/kernel/time.c

@@ -228,7 +228,7 @@ static __init void nios2_clockevent_init(struct device_node *timer)
 	nios2_timer_get_base_and_freq(timer, &iobase, &freq);
 
 	irq = irq_of_parse_and_map(timer, 0);
-	if (irq < 0)
+	if (!irq)
 		panic("Unable to parse timer irq\n");
 
 	nios2_ce.timer.base = iobase;