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.
Signed-off-by: Ben Greear <greearb@candelatech.com>
Link: https://lore.kernel.org/r/20191126214744.1283-1-greearb@candelatech.com
[removed unnecessary check and some unnecessary parentheses]
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>

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

@@ -4911,7 +4911,7 @@ 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);
 	}