WIP: bring back "mmc: sdhci-esdhc-imx: clear DMA_SEL when disable DMA mode"

I saw a few sdhc timeouts on 5.9 but with this I'm rebooting / cold starting without problems for ~15 times now and also it seems to speed up things.

since I don't have a real logical reason for the above revert (though am thankful that sebastian found it), let's bring the original code back.

@sebastian.krzyszkowiak in case you run 5.9, could you test too for some time? thanks a lot!