dmaengine: stm32-dma: fix stm32_dma_prep_slave_sg in case of MDMA chaining

dmaengine: stm32-dma: fix stm32_dma_prep_slave_sg in case of MDMA chaining

Current Target (CT) have to be reset when starting an MDMA chaining use
case, as Double Buffer mode is activated. It ensures the DMA will start
processing the first memory target (pointed with SxM0AR).

Fixes: 723795173ce1 ("dmaengine: stm32-dma: add support to trigger STM32 MDMA")
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20231004155024.2609531-1-amelie.delaunay@foss.st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>