dmaengine: doc: client: Update for dmaengine_get_dma_device() usage

Client drivers should use the dmaengine_get_dma_device(chan) to get the
device pointer which should be used for DMA API for allocations and
mapping.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20201208090440.31792-10-peter.ujfalusi@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>