drivers: rtc: stm32 rtc driver disable alarm interrupt if not used

Note that the Init of the alarm has no need for BCD2BIN conversion,
the LL_RTC_ALMx_Init will do. Add more log debug
The driver must clear the ALARM enable bit before wrting the alarm
registers.

Signed-off-by: Francois Ramu <francois.ramu@st.com>