wifi: mt76: mt7996: fix possible NULL pointer dereference in mt7996_mac_write_txwi()

Fix possible NULL pointer dereference on mvif pointer in
mt7996_mac_write_txwi routine.

Fixes: 15ee62e7

 ("wifi: mt76: mt7996: enable BSS_CHANGED_BASIC_RATES support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/2637628a84f42ad6d7b774e706f041d5b45c8cb5.1685269638.git.lorenzo@kernel.org