[][MAC80211][WiFi6][mt76][Fix dump PFMU tag kernel calltrace]

[Description]
Fix the kernel calltrace when dump PDMU tag content.

For normal mode, it will not allocate memory for test mode data structure mt76_testmode_data.
Before access this data structue, it shall make sure it is not a null
pointer.

[Release-log]
N/A

Change-Id: Id6b18e5f1fdaac5a32c7f7d6ea163fb6bc8a8257
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8215343
1 file changed