mmc: mmc_test: Do not use mmc_start_areq() anymore

The block driver's blk-mq paths do not use mmc_start_areq(). In order to
remove mmc_start_areq() entirely, start by removing it from mmc_test.

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