mmc: mmc: Hold re-tuning in mmc_sleep()

The sleep command is issued after deselecting the
card, but re-tuning won't work on a deselected card
so re-tuning must be held.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>