iwlwifi: mvm: Avoid signal based decisions if ave beacon RSSI is 0

If for some reason statistics notification received from the firmware
reports 0 in average beacon RSSI value, then skip it and avoid signal
based decisions.

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