ath10k: simplify HTC command submitting

The patch removes HTC endpoint tx workers in
favour of direct command submission. This makes a
lot more sense for data path.

mac80211 queues are effectively stopped/woken up
in a more timely fashion preventing build up of
frames. It's possible to push more traffic than
the device/system is able to handle and have no
hiccups or performance degradation with UDP
traffic.

WMI commands will now report errors properly and
possibly block as they actively can wait for tx
credits to become available.

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