wifi: mt76: mt7996: fix NULL pointer dereference in mt7996_mcu_sta_bfer_he

Fix the NULL pointer dereference in mt7996_mcu_sta_bfer_he
routine adding an sta interface to the mt7996 driver.

Found by code review.

Cc: stable@vger.kernel.org
Fixes: 98686cd ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Link: https://patch.msgid.link/20240813081242.3991814-1-make24@iscas.ac.cn
Signed-off-by: Felix Fietkau <nbd@nbd.name>

Author: Ma Ke

drivers/net/wireless/mediatek/mt76/mt7996/mcu.c

@@ -1544,6 +1544,9 @@ mt7996_mcu_sta_bfer_he(struct ieee80211_sta *sta, struct ieee80211_vif *vif,
 	u8 nss_mcs = mt7996_mcu_get_sta_nss(mcs_map);
 	u8 snd_dim, sts;
 
+	if (!vc)
+		return;
+
 	bf->tx_mode = MT_PHY_TYPE_HE_SU;
 
 	mt7996_mcu_sta_sounding_rate(bf);