mmc: dw_mmc: Fix issue with uninitialized dma_slave_config

mmc: dw_mmc: Fix issue with uninitialized dma_slave_config

Depending on the DMA driver being used, the struct dma_slave_config may
need to be initialized to zero for the unused data.

For example, we have three DMA drivers using src_port_window_size and
dst_port_window_size. If these are left uninitialized, it can cause DMA
failures.

For dw_mmc, this is probably not currently an issue but is still good to
fix though.

Fixes: 3fc7eaef44db ("mmc: dw_mmc: Add external dma interface support")
Cc: Shawn Lin <shawn.lin@rock-chips.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Cc: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20210810081644.19353-2-tony@atomide.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>