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