rtc: omap: fix unbalanced clk_prepare_enable/clk_disable_unprepare

There are 2 error paths after clk_prepare_enable() was called, where
clk_disable_unprepare() is missing.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Andreas Platschek <andreas.platschek@opentech.at>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>