clockevents/drivers/mtk: Fix spurious interrupt leading to crash

After analysis done by Yingjoe Chen, the timer appears to have a pending
interrupt when it is enabled.

Fix this by acknowledging the pending interrupt when enabling the timer
interrupt.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Yingjoe Chen <yingjoe.chen@mediatek.com>