mmc: dw_mmc: fix the error handling for dma operation

When dma->start is failed,then it has to fall back to PIO mode
for current transfer.

But Host controller was already set to bits relevant to DMA operation.
If needs to use the PIO mode, Host controller has to stop the DMA
operation. (It's more stable than now.)

When it occurred error, it's not running any request.

Fixes: 3fc7eaef

 ("mmc: dw_mmc: Add external dma interface support")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Cc: <stable@vger.kernel.org> # v4.3+
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>