mt76: fix transmission of encrypted management frames

Hello Felix Fietkau,

The patch 23405236460b: "mt76: fix transmission of encrypted management frames" from Jan 18, 2018, leads to the following static checker warning:

	drivers/net/wireless/mediatek/mt76/mt76x2_tx.c:41 mt76x2_tx()
	warn: always true condition '(wcid->hw_key_idx != -1) => (0-255 != (-1))'

drivers/net/wireless/mediatek/mt76/mt76x2_tx.c

    26  void mt76x2_tx(struct ieee80211_hw *hw, struct ieee80211_tx_control *control,
    27               struct sk_buff *skb)
    28  {
    29          struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
    30          struct mt76x2_dev *dev = hw->priv;
    31          struct ieee80211_vif *vif = info->control.vif;
    32          struct mt76_wcid *wcid = &dev->global_wcid;
    33  
    34          if (control->sta) {
    35                  struct mt76x2_sta *msta;
    36  
    37                  msta = (struct mt76x2_sta *) control->sta->drv_priv;
    38                  wcid = &msta->wcid;
    39          }
    40  
    41          if (vif || (!info->control.hw_key && wcid->hw_key_idx != -1)) {
                                                     ^^^^^^^^^^^^^^^^^^^^^^

We set ->hw_key_idx to -1 but it's a u8 so it gets truncated to 0xFF. This should probably be a define anyway.

    42                  struct mt76x2_vif *mvif;
    43  
    44                  mvif = (struct mt76x2_vif *) vif->drv_priv;
    45                  wcid = &mvif->group_wcid;
    46          }
    47  
    48          mt76_tx(&dev->mt76, control->sta, wcid, skb);
    49  }

regards,
dan carpenter

                                                       ^^^^^^^^^^^^^

Oh, there is another static checker warning because of the "vif" check:

drivers/net/wireless/mediatek/mt76/mt76x2_tx.c:44 mt76x2_tx()
error: we previously assumed 'vif' could be null (see line 41)

Assume "vif" is NULL, and info->control.hw_key is zero and ->hw_key_idx is not -1.

regards,
dan carpenter