wifi: mt76: mt7996: Fix possible oob access in mt7996_mac_write_txwi_80211()

[ Upstream commit 60862846308627e9e15546bb647a00de44deb27b ]

Check frame length before accessing the mgmt fields in
mt7996_mac_write_txwi_80211 in order to avoid a possible oob access.

Fixes: 98686cd21624c ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260226-mt76-addba-req-oob-access-v1-1-b0f6d1ad4850@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>