mmc: mxs-mmc: fix the dma_transfer_direction migration

The commit 05f5799c (mmc-host: move to dma_transfer_direction) left out
the DMA_NONE, in turn breaks the driver as below.

[    0.650000] mxs-mmc mxs-mmc.0: initialized
[    0.650000] mxs-mmc mxs-mmc.1: initialized
[    0.690000] mxs-dma mxs-dma-apbh: maximum bytes for sg entry exceeded: -55906
7475 > 65280
[    0.690000] mxs-mmc mxs-mmc.0: mxs_mmc_ac: failed to prep dma

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>