dmaengine: stm32-dma: fix residue calculation in stm32-dma

In double buffer mode, during residue calculation, the DMA can
automatically switch to the next transfer. Indeed the CT bit that
gives position in the double buffer can has been updated by the
hardware, during calculation.
In this case the SxNDTR register value can not be trusted.
If a transition is detected we consider that the DMA has switched to
the beginning of next sg.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>