wifi: mt76: mt7996: fix insecure data handling of mt7996_mcu_ie_countdown()

Coverity message:
using tainted "hdr->band" variable as an index into an array "(*dev).mt76.phys".
Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1527797 ("Insecure data handling")
Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

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

@@ -412,6 +412,9 @@ mt7996_mcu_ie_countdown(struct mt7996_dev *dev, struct sk_buff *skb)
 	struct header *hdr = (struct header *)data;
 	struct tlv *tlv = (struct tlv *)(data + 4);
 
+	if (hdr->band >= ARRAY_SIZE(dev->mt76.phys))
+		return;
+
 	if (hdr->band && dev->mt76.phys[hdr->band])
 		mphy = dev->mt76.phys[hdr->band];