ath9k: fix BUG_ON triggered by PAE frames

When I initially stumbled upon sequence number problems with PAE frames
in ath9k, I submitted a patch to remove all special cases for PAE
frames and let them go through the normal transmit path.
Out of concern about crypto incompatibility issues, this change was
merged instead:

commit 6c8afef5


Author: Sujith <Sujith.Manoharan@atheros.com>
Date:   Tue Feb 9 10:07:00 2010 +0530

    ath9k: Fix sequence numbers for PAE frames

After a lot of testing, I'm able to reliably trigger a driver crash on
rekeying with current versions with this change in place.
It seems that the driver does not support sending out regular MPDUs with
the same TID while an A-MPDU session is active.
This leads to duplicate entries in the TID Tx buffer, which hits the
following BUG_ON in ath_tx_addto_baw():

    index  = ATH_BA_INDEX(tid->seq_start, bf->bf_seqno);
    cindex = (tid->baw_head + index) & (ATH_TID_MAX_BUFS - 1);

    BUG_ON(tid->tx_buf[cindex] != NULL);

I believe until we actually have a reproducible case of an
incompatibility with another AP using no PAE special cases, we should
simply get rid of this mess.

This patch completely fixes my crash issues in STA mode and makes it
stay connected without throughput drops or connectivity issues even
when the AP is configured to a very short group rekey interval.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>