iwlwifi: mvm: Fix avg-power report

On AX200, the average power was showing positive instead of negative, but
otherwise matched the expected RSSI.  Flip the value to negative before
 giving to mac80211.

https://git.kernel.org/pub/scm/linux/kernel/git/iwlwifi/iwlwifi-next.git/commit/?id=88ad368a84d8df81c7b832a6aba3487c63019b29

drivers/net/wireless/intel/iwlwifi/mvm/mac80211.c

@@ -4750,7 +4750,7 @@ static void iwl_mvm_mac_sta_statistics(struct ieee80211_hw *hw,
  struct iwl_mvm_sta *mvmsta = iwl_mvm_sta_from_mac80211(sta);
 
  if (mvmsta->avg_energy) {
- sinfo->signal_avg = mvmsta->avg_energy;
+ sinfo->signal_avg = -(s8)mvmsta->avg_energy;
  sinfo->filled |= BIT_ULL(NL80211_STA_INFO_SIGNAL_AVG);
  }