This also fixes a possible race condition when exiting initialisation mode:
reading then writing to ISR (via ISR &= ~RTC_ISR_INIT) will clear any flags
that were set by the hardware between the read and the write. The correct
way to clear just the INIT bit is to just do a single write via ISR =
~RTC_ISR_INIT, which will not clear any other flags (they must be written
to 0 to clear), and that is exactly what LL_RTC_DisableInitMode does.
Signed-off-by: Damien George <damien@micropython.org>