ath6kl: prevent potential array overflow in ath6kl_add_new_sta()

The value for "aid" comes from skb->data so Smatch marks it as
untrusted.  If it's invalid then it can result in an out of bounds array
access in ath6kl_add_new_sta().

Fixes: 572e27c0 ("ath6kl: Fix AP mode connect event parsing and TIM updates")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200813141315.GB457408@mwanda