dmaengine: ste_dma40: fix implicit conversion

The function d40_prep_sg takes the type enum dma_transfer_direction
as second last parameter. However, the memcpy calls pass DMA_NONE
which is of type enum dma_data_direction. Fix this by passing the
actual transfer direction DMA_MEM_TO_MEM.

This does not change the actual code flow since only the transfer
direction DMA_MEM_TO_DEV and DMA_DEV_TO_MEM are actually used in the
function d40_prep_sg.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>