mpam: Code security rectification

hulk inclusion
category: feature
feature: ARM MPAM support
bugzilla: 48265
CVE: NA

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Two aspects need to change:
1. Change the function "resctrl_ctrlmon_disable" from int to void
and delete unuse variable.
2. Change the code that refrences a null return value.Dereferencing
"rdtgrp", which is known to be "NULL".
Signed-off-by: Hongbo Yao <yaohongbo@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Reviewed-by: Cheng Jian <cj.chengjian@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>