dmaengine: stm32-dma: clean channel configuration when channel is freed

dmaengine: stm32-dma: clean channel configuration when channel is freed

When dma_channel_release is called, it means that the channel won't be used
anymore with the configuration it had. To ensure a future client can safely
use the channel after it has been released, clean the configuration done
when channel was requested.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20201120143320.30367-3-amelie.delaunay@st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>