clocksource: exynos_mct: silence a static checker warning

My guess is we aren't going to have a 2 digit cpuid here any time soon
but the static checkers don't know that and complain that the snprintf()
could overflow.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>