mmc: core: Move calls to ->prepare_hs400_tuning() closer to mmc code

Move the calls to ->prepare_hs400_tuning(), from mmc_retune() into
mmc_hs400_to_hs200(), as it better belongs there, rather than being generic
to all type of cards.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>