ath10k: simplify txi lifespan

Up until now we were using two booleans to keep
track of htt and htc tx completions. These can
come in no specific order.

Keep in mind that it was safe to access those
without any locks since tx completion processing
is done one endpoint at a time (that's the current
PCI backend implementation).

The new approach uses refcount. Since we need 2
completions we setup refcount=2 for mgmt and
data tx. This also depends on the tx completion
behaviour.

If the PCI tx completion scheme ever changes we'll
need locking to synchronize tx completions for
htt.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
ath10k: merge htt_tx_info into ath10k_skb_cb

This eliminates the wrapper structure htt_tx_info.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>