driver: counter: counter_ll_stm32_rtc.c: Add 1 tick to alarm

Add +1 tick to alarm in order to compensate the partially started tick.
Alarm will expire between requested ticks and ticks+1.
In case only 1 tick is requested, it will avoid that +1 Tick event
occurs before alarm setting is finished.

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>