mmc: sdhci-of-esdhc: exit HS400 properly before setting any speed mode

The eSDHC HS400 timing requires many specific registers setting,
unlike other speed modes which need to set only host controller 2
register. When driver needs to downgrade HS400 mode to other speed
mode, the controller have to exit HS400 timing properly first.
This patch is to support the procedure of HS400 exiting at the
beginning of esdhc_set_uhs_signaling.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20200522031256.856-1-yangbo.lu@nxp.com


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