[][MAC80211][mt76][Add monitor vif check in testmode]

[Description]
Add monitor vif check in testmode to avoid kernel crash
when monitor interface is not up.

[Release-log]
N/A

Change-Id: I06460b0f6c7d93352ffb4d9f275aedee027c9105
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7197597
5 files changed