clocksource/drivers/exynos_mct: Change exynos4_mct_tick_clear return type to void

Return value of exynos4_mct_tick_clear() was never checked so it can
be safely changed to void.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>