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: NBen Greear <greearb@candelatech.com>
Signed-off-by: NMichal Kazior <michal.kazior@tieto.com>
Signed-off-by: NKalle Valo <kvalo@qca.qualcomm.com>