drivers: dma stm32 keep the channel busy if overriden

When the channel is overriden by the HAL,
the dma irq must always be handled (even if not busy).
Only when the dma channel is not overriden by the HAL
the irq is exiting when the channel is no more busy.

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