mt76x2: apply coverage class on slot time too

According to 802.11-2007 17.3.8.6 (slot time), the slot time should
be increased by 3 us * coverage class. Taking into account coverage
class in slot time configuration allows to increase by an order of
magnitude the throughput on a 4Km link in a noisy environment
Tested-by: NLuca Bisti <luca.bisti@gmail.com>
Tested-by: NGaetano Catalli <gaetano.catalli@gmail.com>
Signed-off-by: NLorenzo Bianconi <lorenzo.bianconi@redhat.com>
Acked-by: NFelix Fietkau <nbd@nbd.name>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/mediatek/mt76/mt76x2_init.c

@@ -482,7 +482,10 @@ void mt76x2_set_tx_ackto(struct mt76x2_dev *dev)
 {
 	u8 ackto, sifs, slottime = dev->slottime;
 
+	/* As defined by IEEE 802.11-2007 17.3.8.6 */
 	slottime += 3 * dev->coverage_class;
+	mt76_rmw_field(dev, MT_BKOFF_SLOT_CFG,
+		       MT_BKOFF_SLOT_CFG_SLOTTIME, slottime);
 
 	sifs = mt76_get_field(dev, MT_XIFS_TIME_CFG,
 			      MT_XIFS_TIME_CFG_OFDM_SIFS);

drivers/net/wireless/mediatek/mt76/mt76x2_main.c

@@ -247,8 +247,7 @@ mt76x2_bss_info_changed(struct ieee80211_hw *hw, struct ieee80211_vif *vif,
 		int slottime = info->use_short_slot ? 9 : 20;
 
 		dev->slottime = slottime;
-		mt76_rmw_field(dev, MT_BKOFF_SLOT_CFG,
-			       MT_BKOFF_SLOT_CFG_SLOTTIME, slottime);
+		mt76x2_set_tx_ackto(dev);
 	}
 
 	mutex_unlock(&dev->mutex);