wifi: mt76: mt7996: Fix possible NULL pointer dereference in mt7996_mac_write_txwi_80...

wifi: mt76: mt7996: Fix possible NULL pointer dereference in mt7996_mac_write_txwi_80211()

For injected frames (e.g. via radiotap), mac80211 can pass
info->control.vif = NULL, as explicitly noted in struct ieee80211_tx_info.
Check vif pointer before executing ieee80211_vif_is_mld() in
mt7996_mac_write_txwi_80211 routine in order to avoid a possible NULL
pointer dereference.

Fixes: f0b0b239b8f36 ("wifi: mt76: mt7996: rework mt7996_mac_write_txwi() for MLO support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260531-mt7996_mac_write_txwi_80211-null-ptr-deref-v1-1-6dd38e1d3422@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>