mmc: sdhci: Fix issue with uninitialized dma_slave_config

mmc: sdhci: 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 at least if external TI SDMA is ever configured for sdhci.

For other external DMA cases, this is probably not currently an issue but
is still good to fix though.

Fixes: 18e762e3b7a7 ("mmc: sdhci: add support for using external DMA devices")
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Chunyan Zhang <zhang.chunyan@linaro.org>
Cc: Faiz Abbas <faiz_abbas@ti.com>
Cc: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Cc: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Link: https://lore.kernel.org/r/20210810081644.19353-1-tony@atomide.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>