ath10k: drain tx before restarting hw

This makes sure no further tx requests are
submitted to HTT before driver teardown.

This should prevent invalid pointer/NULL
dereference on htt tx pool in ath10k_htt_tx() in
some cases of heavy traffic.

kvalo: remove the WARN_ON() if conf_mutex is held

Reported-By: Ben Greear <greearb@candelatech.com>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>