rtc: stm32: add missed clk_disable_unprepare in error path of resume

The resume() forgets to call clk_disable_unprepare() when failed.
Add the missed call to fix it.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Reviewed-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20191205160655.32188-1-hslester96@gmail.com


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