Fix wrong memory limit calculation in memory-test

If the length of the memory address range passed to the "mtest" command is
not of the form 2^x - 1, not all address lines are tested. This bug is
inherited from the original software at
http://www.netrino.com/Embedded-Systems/How-To/Memory-Test-Suite-C

. Fix
this.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>