ath10k: remove TX lock from ath10k_htt_tx_inc_pending

This commit removes the call to ath10k_mac_tx_lock() from
ath10k_htt_tx_inc_pending() in case the high water mark is reached.

ath10k_mac_tx_lock() calls ieee80211_stop_queues() in order to stop
mac80211 from pushing more TX data to the driver (this is the TX lock).

If a driver is trying to fetch an skb from a queue while the queue is
stopped, ieee80211_tx_dequeue() will return NULL.

So, in ath10k_mac_tx_push_txq(), there is a risk that the call to
ath10k_htt_tx_inc_pending() results in a stop of the mac80211 TX queues
just before the skb is fetched.

This will cause ieee80211_tx_dequeue() to return NULL and
ath10k_mac_tx_push_txq() to exit prematurely and return -ENOENT.
Before the function returns ath10k_htt_tx_dec_pending() will be called.
This call will re-enable the TX queues through ath10k_mac_tx_unlock().
When ath10k_mac_tx_push_txq() has returned, the TX queue will be
returned back to mac80211 with ieee80211_return_txq() without the skb
being properly consumed.

Since the TX queues were re-enabled in the error exit path of
ath10k_mac_tx_push_txq(), mac80211 can continue pushing data to the
driver. If the hardware does not consume the data, the above mentioned
case will be repeated over and over.

A case when the hardware is not able to transmit the data from the host
is when a STA has been dis-associated from an AP and has not yet been
able to re-associate. In this case there will be no TX_COMPL_INDs from
the hardware, resulting in the TX counter not be decremented.

This phenomenon has been observed in both a real and a test setup.

In order to fix this, the actual TX locking (the call to
ath10k_mac_tx_lock()) was removed from ath10k_htt_tx_inc_pending().
Instead, ath10k_mac_tx_lock() is called separately after the skb has
been fetched (after the call to ieee80211_tx_dequeue()). At this point
it is OK to stop the queues.

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Patchwork-Id: 11112997
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>