mtd: tests: fix integer overflow issues

These multiplications are done with 32-bit arithmetic, then converted to
64-bit. We should widen the integers first to prevent overflow. This
could be a problem for large (>4GB) MTD's.

Detected by Coverity.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Akinobu Mita <akinobu.mita@gmail.com>