clocksource/drivers/tango-xtal: Fix boot hang due to incorrect test

Commit 0881841f7e introduced a regression by inverting a test check
after calling clocksource_mmio_init(). That results on the system to
hang at boot time.

Fix it by inverting the test again.

Fixes: 0881841f7e ("Replace code by clocksource_mmio_init")
Reported-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/tango_xtal.c

@@ -42,7 +42,7 @@ static void __init tango_clocksource_init(struct device_node *np)
 
 	ret = clocksource_mmio_init(xtal_in_cnt, "tango-xtal", xtal_freq, 350,
 				    32, clocksource_mmio_readl_up);
-	if (!ret) {
+	if (ret) {
 		pr_err("%s: registration failed\n", np->full_name);
 		return;
 	}