clocksource/drivers/sp804: Remove a mismatched comment

writel(0, base + TIMER_CTRL);
... ...
writel(xxx | TIMER_CTRL_PERIODIC, base + TIMER_CTRL);

The timer is just temporarily disabled, and it will be set to periodic
mode later.

The description of the field TimerMode of the register TimerXControl
as shown below:
0 = Timer module is in free-running mode (default)
1 = Timer module is in periodic mode.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200918132237.3552-5-thunder.leizhen@huawei.com