cmd_mem.c: Update 'iteration_limit' to unsigned long

With e37f1eb 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>

common/cmd_mem.c

@@ -999,7 +999,7 @@ static int do_mem_mtest(cmd_tbl_t *cmdtp, int flag, int argc,
 {
 	ulong start, end;
 	vu_long *buf, *dummy;
-	int iteration_limit = 0;
+	ulong iteration_limit = 0;
 	int ret;
 	ulong errs = 0;	/* number of errors, or -1 if interrupted */
 	ulong pattern = 0;