platform_execute_tuning should not really exist as it does not
do anything useful.

So remove this ops and directly plug sdhci_msm_execute_tuning
with mmc_host_ops.

Also in case of HS400 tuning clear SDHCI_HS400_TUNING flag once
HS400 related mode selection is done.
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>