mmc_test: change simple_strtol() to strict_strtol()

It's better to use strict_strtol() to convert user's input and strictly
check it. At least it forbids to interpret wrong input as a 0 and
prevents to run all tests.

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Chris Ball <cjb@laptop.org>