iwlwifi: mvm: BT Coex - Correctly enable and treat rssi events

Rssi events were enabled on interfaces using 5.2GHz.
Interfaces on 5.2GHz were taken into account while
determining the ACK / CTS kill mask. Fix that.

The last rssi notified to BT Coex was reset every BT Coex
Notification. Since we get a lot of these notifications
from the firmware, we reset the rssi all the time which
means that the bt_rssi_event is called all the time.
Fix that by puting the rssi we pull upon BT Coex
notification into iwl_mvm_vif_bf_data.last_bt_coex_event

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>