ath9k: Fix potential out of bounds in ath9k_htc_txcompletion_cb()

The value of "htc_hdr->endpoint_id" comes from skb->data so Smatch marks
it as untrusted so we have to check it before using it as an array
offset.

This is similar to a bug that syzkaller found in commit e4ff08a4
("ath9k: Fix use-after-free Write in ath9k_htc_rx_msg") so it is
probably a real issue.

Fixes: fb9987d0 ("ath9k_htc: Support for AR9271 chipset.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200813141253.GA457408@mwanda

drivers/net/wireless/ath/ath9k/htc_hst.c

@@ -339,6 +339,8 @@ void ath9k_htc_txcompletion_cb(struct htc_target *htc_handle,
 
 	if (skb) {
 		htc_hdr = (struct htc_frame_hdr *) skb->data;
+		if (htc_hdr->endpoint_id >= ARRAY_SIZE(htc_handle->endpoint))
+			goto ret;
 		endpoint = &htc_handle->endpoint[htc_hdr->endpoint_id];
 		skb_pull(skb, sizeof(struct htc_frame_hdr));