mmc: core: Export a few functions needed for blkmq support

The following functions are needed by the mmc block device driver, once it
converts to blkmq, therefore let's export them.

mmc_start_bkops()
mmc_start_request()
mmc_retune_hold_now()
mmc_retune_release()

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>