ath10k: wake up offchannel queue properly

Once HTT Tx queue got full offchannel queue was
stopped and never woken up again. This broke, e.g.
P2P. This could be reproduced after running a lot
of traffic enough to saturate 100% of the driver
Tx queue and then trying to send offchannel
traffic.

Fixes: 96d828d4 ("ath10k: rework tx queue locking")
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>