dmaengine: imx-dma: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: arm):

drivers/dma/imx-dma.c: In function ‘imxdma_xfer_desc’:
drivers/dma/imx-dma.c:542:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (slot == IMX_DMA_2D_SLOT_A) {
      ^
drivers/dma/imx-dma.c:559:2: note: here
  case IMXDMA_DESC_MEMCPY:
  ^~~~

Notice that, in this particular case, the code comment is
modified in accordance with what GCC is expecting to find.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20190729225221.GA24269@embeddedor


Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>