drivers: dma stm32U5 GPDMA keeps the channel busy if overriden

For the stm32U5 serie, the busy flag is handled as other serie.
When the DMA is overriden by other HAL drivers, the busy flag
is not considered.
Refer to https://github.com/zephyrproject-rtos/zephyr/pull/47645


There is no Mux for this GPDMA and fixed 16 channels.

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