iwlwifi: mvm: set keepalive period regardless of PM state

The firmware starts sending nulldata frames for keepalive immediately
after association, regardless of power management state. The driver
thus needs to configure keep alive period unconditionally.

Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>