iwlwifi: Check BSS ctx active before call mac80211

It is possible that the BSS context is not active (for example
when the current mode is set to GO), or that the vif->type is
different than station. In such a case we cannot
call mac80211 to report the average rssi for the interface
(the function assumes that the vif is valid and that the type
is station).

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>