mmc: mmc_test: replace strict_strtol() with kstrtol_from_user()

The usage of strict_strtol() is not preferred, because
strict_strtol() is obsolete. Thus, kstrtol() should be used.

Also, both kstrtol() and copy_from_user() can be replaced
with kstrtol_from_user() to make the code simpler.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>