CVE-2026-74322: wifi: mt76: mt7996: Fix possible NULL pointer dereference in mt7996_mac_write_txwi_80211()

In the Linux kernel, the following vulnerability has been resolved:

wifi: mt76: mt7996: Fix possible NULL pointer dereference in mt7996macwritetxwi80211()

For injected frames (e.g. via radiotap), mac80211 can pass info->control.vif = NULL, as explicitly noted in struct ieee80211txinfo. Check vif pointer before executing ieee80211vifismld() in mt7996macwritetxwi80211 routine in order to avoid a possible NULL pointer dereference.