iwlwifi: mvm: set assoc_beacon_arrive_time

When updating the mac context after association,
assoc_beacon_arrive_time is not being set, which causes the FW to
set a wrong TSF to the MAC.

Fix this by setting the assoc_beacon_arrive_time when updating the
mac context after association.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>