mmc: mmc: Introduce mmc_abort_tuning()

If a tuning command times out, the card could still be processing it, which
will cause problems for recovery. The eMMC specification says that CMD12
can be used to stop CMD21, so add a function that does that.

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