mmc: test: clean up mmc_test_cleanup()

mmc_test_cleanup() has same body as __mmc_test_prepare() with write
except the character to memset().

Signed-off-by: Yue Hu <huyue2@yulong.com>
Link: https://lore.kernel.org/r/20210119073705.375-1-zbestahu@gmail.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>