drivers: rtc: rtc_ll_stm32: coverity 368806 fix

Replace DIV_ROUND_CLOSEST() with a normal division to simplify
operation and avoid redundant handling of signed integer rounding
as the temp value is an unsigned value. The temp value is in nano
precision, so rounding is largely inconsequencial anyway compared
to the comparatively low precision sync prescaler.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>