rtc: snvs: Remove NULL pointer check before clk_*

Because clk_* already checked NULL clock parameter,
so the additional checks are unnecessary, just remove them.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201113080305.65961-1-vulab@iscas.ac.cn