mmc: moxart: Fix issue with uninitialized dma_slave_config

mmc: moxart: 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 moxart, this is probably not currently an issue but is still good to
fix though.

Fixes: 1b66e94e6b99 ("mmc: moxart: Add MOXA ART SD/MMC driver")
Cc: Jonas Jensen <jonas.jensen@gmail.com>
Cc: Vinod Koul <vkoul@kernel.org>
Cc: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20210810081644.19353-3-tony@atomide.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>