mac80211: fix driver RSSI event calculations

The ifmgd->ave_beacon_signal value cannot be taken as is for
comparisons, it must be divided by since it's represented
like that for better accuracy of the EWMA calculations. This
would lead to invalid driver RSSI events. Fix the used value.

Fixes: 615f7b9b ("mac80211: add driver RSSI threshold events")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>