mmc: sdhci-esdhc-imx: clear DMA_SEL when disable DMA mode

Currently, when use standard tuning, driver default disable DMA just before
send tuning command. But on i.MX8 usdhc, this is not enough. Need also clear
DMA_SEL. If not, once the DMA_SEL select AMDA2 before, even dma already disabled,
when send tuning command, usdhc will still prefetch the ADMA script from wrong
DMA address, then we will see IOMMU report some error which show lack of TLB
mapping.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/1582100757-20683-7-git-send-email-haibo.chen@nxp.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>