ath10k: improve channel switching

In some cases during heavy tx vdev stop-start
would timeout on vdev synchronization causing
traffic to stall for a few seconds.

Instead of stop-starting use a dedicated vdev
restart command and down vdevs explicitly before
doing so.

This gets rid of the synchronization
warnings/timeouts and makes channel switching
smoother during traffic.

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