cmd_mem.c: Update 'iteration_limit' to unsigned long

With e37f1eb4 we now use strict_strtoul() in do_mem_mtest() and this
gives us a warning:
../include/vsprintf.h:38:5: note: expected 'long unsigned int *' but
argument is of type 'int *'

Signed-off-by: Tom Rini <trini@konsulko.com>