mt76: mt7915: fix possible NULL pointer dereference in mt7915_mac_fill_rx_vector

Fix possible NULL pointer dereference in mt7915_mac_fill_rx_vector
routine if the chip does not support dbdc and the hw reports band_idx
set to 1.

Fixes: 78fc30a2 ("mt76: mt7915: move testmode data from dev to phy")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7915/mac.c

@@ -864,8 +864,11 @@ mt7915_mac_fill_rx_vector(struct mt7915_dev *dev, struct sk_buff *skb)
 	int i;
 
 	band_idx = le32_get_bits(rxv_hdr[1], MT_RXV_HDR_BAND_IDX);
-	if (band_idx && !phy->band_idx)
+	if (band_idx && !phy->band_idx) {
 		phy = mt7915_ext_phy(dev);
+		if (!phy)
+			goto out;
+	}
 
 	rcpi = le32_to_cpu(rxv[6]);
 	ib_rssi = le32_to_cpu(rxv[7]);
@@ -890,8 +893,8 @@ mt7915_mac_fill_rx_vector(struct mt7915_dev *dev, struct sk_buff *skb)
 
 	phy->test.last_freq_offset = foe;
 	phy->test.last_snr = snr;
+out:
 #endif
-
 	dev_kfree_skb(skb);
 }