mac80211: inform wireless layer when frame RSSI is invalid

When the low-level driver returns an invalid RSSI indication,
set the signal value to 0 as an indication to the upper layer.

Also, skip average level computation if signal is invalid.

Signed-off-by: Jean Pierre TOSONI <jp.tosoni@acksys.fr>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>