drivers: timer: stm32u5 lptimer waits for DIER complete

On the stm32U5, when modifying the DIER register of the LPTIM peripheral,
a new write operation to can only be performed when the previous write
operation is completed and before going-on.
This is done with a function call for better readability.

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