clocksource/drivers/timer-of: Fix invalid iomap check

A typo in the code checks the return value of iomap against !NULL
and, thus, fails everytime the mapping succeed.

Fix this by inverting the condition in the check.
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/timer-of.c

@@ -120,7 +120,7 @@ static __init int timer_base_init(struct device_node *np,
 	const char *name = of_base->name ? of_base->name : np->full_name;
 
 	of_base->base = of_io_request_and_map(np, of_base->index, name);
-	if (of_base->base) {
+	if (!of_base->base) {
 		pr_err("Failed to iomap (%s)\n", name);
 		return -ENXIO;
 	}