mt76: mt7615: reduce maximum VHT MPDU length to 7991

After fixing mac80211 to allow larger A-MSDUs in some cases, there have been
reports of performance regressions and packet loss with some clients.
It appears that the issue occurs when the hardware is transmitting A-MSDUs
bigger than 8k. Limit the local VHT MPDU size capability to 7991, matching
the value used for MT7915 as well.
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200923052442.24141-1-nbd@nbd.name

drivers/net/wireless/mediatek/mt76/mt7615/init.c

@@ -460,7 +460,7 @@ void mt7615_init_device(struct mt7615_dev *dev)
 	dev->mphy.sband_2g.sband.ht_cap.cap |= IEEE80211_HT_CAP_LDPC_CODING;
 	dev->mphy.sband_5g.sband.ht_cap.cap |= IEEE80211_HT_CAP_LDPC_CODING;
 	dev->mphy.sband_5g.sband.vht_cap.cap |=
-			IEEE80211_VHT_CAP_MAX_MPDU_LENGTH_11454 |
+			IEEE80211_VHT_CAP_MAX_MPDU_LENGTH_7991 |
 			IEEE80211_VHT_CAP_MAX_A_MPDU_LENGTH_EXPONENT_MASK;
 	mt7615_cap_dbdc_disable(dev);
 	dev->phy.dfs_state = -1;