mmc: sdhci: add delay after the last tuning command

When host set the host->tuning_delay, even the last tuning
command need a delay, otherwise the first command after the
tuning will meet issue.

Take i.MX7D as an example, there will be the following log:
  mmc2: switch to high-speed from hs200 failed, err:-110
  mmc2: error -110 whilst initialising MMC card

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>