wcn36xx: Update dtim period before starting BSS

The dtim period sent to FW was 0 because the dtim period
was never set. This caused an incorrect dtim count to be sent in
beacons.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>