wifi: mac80211: mlme: do IEEE80211_STA_RESET_SIGNAL_AVE per link

Remove the IEEE80211_STA_RESET_SIGNAL_AVE flag and use
a bool instead, but invert the polarity (now calling it
tracking_signal_avg) so we don't have to initialize it,
and put that into the link instead.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>