mmc: core: Simplify by adding mmc_execute_tuning()

For each MMC, SD and SDIO there is code that
holds the clock, calls ops->execute_tuning, and
releases the clock. Simplify the code a bit by
providing a separate function to do that.

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