clocksource/drivers/sp804: Correct clk_get_rate handle

clk_get_rate won't return negative value, correct clk_get_rate handle.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201029123317.90286-4-wangkefeng.wang@huawei.com