mmc: core: Add mmc_retune_hold_now()

mmc_return_hold() / mmc_retune_release() are used around a group of
commands to prevent re-tuning between the commands. Re-tuning can still
happen before the first command. In some cases, re-tuning must be
prevented entirely. Add mmc_retune_hold_now() for that purpose. It is
added in preparation for CQE support where it will be used by CQE recovery.

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