mmc: mmc_test: Uninitialized return value

We never set "ret" to RESULT_OK.

Fixes: 9f9c4180 ("mmc: mmc_test: add test for non-blocking transfers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>