mt76: mt7915: fix WMM index on DBDC cards

WMM index range needs to be split between both PHYs if a second PHY exists.
The condition for that was accidentally written as checking if the vif PHY
is the secondary one
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7915/main.c

@@ -215,7 +215,7 @@ static int mt7915_add_interface(struct ieee80211_hw *hw,
 	mvif->phy = phy;
 	mvif->band_idx = ext_phy;
 
-	if (ext_phy)
+	if (dev->mt76.phy2)
 		mvif->wmm_idx = ext_phy * (MT7915_MAX_WMM_SETS / 2) +
 				mvif->idx % (MT7915_MAX_WMM_SETS / 2);
 	else