ath10k: flush hif buffers before recovery

Transport buffers weren't flushed and processed
before queueing hw recovery request to mac80211.

This could in theory result in an unwanted htt/wmi
rx events being processed while mac80211 recovers
the device and possibly interfere or even crash
the system.

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