Fix missing return in do_mem_loop()

For some reason this does not normally cause a compiler warning, but the code
seems to be incorrect. Add the missing return.

Signed-off-by: Simon Glass <sjg@chromium.org>

common/cmd_mem.c

@@ -551,6 +551,8 @@ static int do_mem_loop(cmd_tbl_t *cmdtp, int flag, int argc,
 			*cp++;
 	}
 	unmap_sysmem(buf);
+
+	return 0;
 }
 
 #ifdef CONFIG_LOOPW