rtc: brcmstb-waketimer: Remove redundant null check before clk_disable_unprepare

Because clk_disable_unprepare() already checked NULL clock parameter,
so the additional check is unnecessary, just remove it.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20201113074538.65028-1-vulab@iscas.ac.cn