iwlwifi: mvm: change the parameters for calculating an AP TBTT

Change the parameters for calculating an AP TBTT to 64/36 instead of
80/20, to increase the interval between a station vif and an AP
vif TBTT events.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>