drivers: dma for stm32u5 returns from a repeated start or stop command

On a repeated dma_start, the stm3u5 dma driver should return
immediately if it is already started.
On a repeated dma_stop, the stm3u5 dma driver should return
immediately if it is already stopped.
This like the dma driver does for other stm32 series.

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