ath10k: fix dtim period with beacon templates

Firmware supporting beacon templates (i.e. wmi-tlv
for qca6174) doesn't implicitly take dtim period
from the template. Instead it requires vdev param
to be set accordingly.

This fixes dtim period being stuck at 3.

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