rtc: coh901331: fix a missing check of clk_prepare

clk_prepare() could fail, so let's check its status and if it fails
return its error code upstream.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>