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

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