mmc: mmc_test: make performance test area size about 4MiB

The test area size was set to the preferred erase size but for comparison
purposes it is better if it is the same size for different devices.  Make
it a multiple of preferred erase size that is greater than or equal to 4MiB.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>