mmc: mmc_test: Fix "Commands during non-blocking write" tests

mmc_test_check_result_async() requires that struct mmc_async_req is
contained within struct mmc_test_async_req. Fix the "Commands during
non-blocking write" tests so that is the case.

Fixes: 4bbb9aac ("mmc: mmc_test: Add tests for sending commands during transfer")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Ritesh Harjani <riteshh@codeaurora.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>