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