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