ath10k: make monitor vdev down before stoping it

Following sequence causes FW crash:
-monitor vdev up,
-monitor vdev stop,
-monitor vdev delete.
Making monitor vdev down before stoping it works ok:
-monitor vdev up,
-monitor vdev down,
-monitor vdev stop,
-monitor vdev delete.

Signed-off-by: Marek Puzyniak <marek.puzyniak@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>