rtc: s35390a: Remove the unneeded result variable

Return the value s35390a_set_reg() directly instead of storing it in
another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Link: https://lore.kernel.org/r/20220905090119.335121-1-ye.xingchen@zte.com.cn


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>