mmc: mmc_test: Use kcalloc() in mmc_test_alloc_mem()

* The script "checkpatch.pl" pointed information out like the following.

  WARNING: Prefer kcalloc over kzalloc with multiply

  Thus fix the affected source code place.

* Replace the specification of a data structure by a pointer dereference
  to make the corresponding size determination a bit safer according to
  the Linux coding style convention.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>