drivers: stm32-rtc: return -ENODATA when RTC is uninitialized

rtc_get_time() on STM32 does not implement the -ENODATA return code.
This prevents testing the initialisation status of the RTC.

Fixed by reading INITS flag and adding a error path in
rtc_stm32_get_time().

Signed-off-by: Adrien Bruant <adrien.bruant@aalberts-hfc.com>