ath10k: stop monitor vdev for sta assoc

This prevents some fw revisions from crashing in
many cases when user is trying to run a
promiscuous station interface (e.g. sniffing,
4addr bridge).

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