mt76: mt7921: fix the dwell time control

dwell time for the scan is not configurable according to the current
firmware submitted into linux-firmware.git, so leave the dwell time 0 to
indicate the dwell time always determined by the firmware.

Fixes: 399090ef ("mt76: mt76_connac: move hw_scan and sched_scan routine in mt76_connac_mcu module")
Suggested-by: Soul Huang <Soul.Huang@mediatek.com>
Co-developed-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt76_connac_mcu.c

@@ -1332,7 +1332,7 @@ int mt76_connac_mcu_hw_scan(struct mt76_phy *phy, struct ieee80211_vif *vif,
 {
 	struct mt76_vif *mvif = (struct mt76_vif *)vif->drv_priv;
 	struct cfg80211_scan_request *sreq = &scan_req->req;
-	int n_ssids = 0, err, i, duration = MT76_CONNAC_SCAN_CHANNEL_TIME;
+	int n_ssids = 0, err, i, duration;
 	int ext_channels_num = max_t(int, sreq->n_channels - 32, 0);
 	struct ieee80211_channel **scan_list = sreq->channels;
 	struct mt76_dev *mdev = phy->dev;
@@ -1369,6 +1369,7 @@ int mt76_connac_mcu_hw_scan(struct mt76_phy *phy, struct ieee80211_vif *vif,
 	req->ssid_type_ext = n_ssids ? BIT(0) : 0;
 	req->ssids_num = n_ssids;
 
+	duration = is_mt7921(phy->dev) ? 0 : MT76_CONNAC_SCAN_CHANNEL_TIME;
 	/* increase channel time for passive scan */
 	if (!sreq->n_ssids)
 		duration *= 2;