clocksource/drivers/mtk_timer: Fix memleak in mtk_timer_init()

Add error path to clear evt struct allocated by kzalloc() in the beginning of
function mtk_timer_init().

Acked-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Alexey Klimov <alexey.klimov@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>