mt76: Fix comparisons with invalid hardware key index

With gcc 4.1.2:

    drivers/net/wireless/mediatek/mt76/mt76x0/tx.c: In function ‘mt76x0_tx’:
    drivers/net/wireless/mediatek/mt76/mt76x0/tx.c:169: warning: comparison is always true due to limited range of data type
    drivers/net/wireless/mediatek/mt76/mt76x2_tx_common.c: In function ‘mt76x2_tx’:
    drivers/net/wireless/mediatek/mt76/mt76x2_tx_common.c:35: warning: comparison is always true due to limited range of data type

While assigning -1 to a u8 works fine, comparing with -1 does not work
as expected.

Fix this by comparing with 0xff, like is already done in some other
places.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/mediatek/mt76/mt76x0/tx.c

@@ -166,7 +166,7 @@ void mt76x0_tx(struct ieee80211_hw *hw, struct ieee80211_tx_control *control,
 	if (sta) {
 		msta = (struct mt76_sta *) sta->drv_priv;
 		wcid = &msta->wcid;
-	} else if (vif && (!info->control.hw_key && wcid->hw_key_idx != -1)) {
+	} else if (vif && (!info->control.hw_key && wcid->hw_key_idx != 0xff)) {
 		struct mt76_vif *mvif = (struct mt76_vif *)vif->drv_priv;
 
 		wcid = &mvif->group_wcid;

drivers/net/wireless/mediatek/mt76/mt76x2_tx_common.c

@@ -32,7 +32,7 @@ void mt76x2_tx(struct ieee80211_hw *hw, struct ieee80211_tx_control *control,
 		msta = (struct mt76x2_sta *)control->sta->drv_priv;
 		wcid = &msta->wcid;
 		/* sw encrypted frames */
-		if (!info->control.hw_key && wcid->hw_key_idx != -1)
+		if (!info->control.hw_key && wcid->hw_key_idx != 0xff)
 			control->sta = NULL;
 	}