drivers: clock_control: stm32: release Backup Domain access refcount

Fix STM32 WBA and H7 clock drivers to release the reference counter
added to access LSE configuration controllers once the clock is
configured. Keeping such an unbalanced access request is no more
needed since SoC functions manage reference counting of access requests.

Fixes issue 90942.

Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>