wifi: rtw88: usb: Enable RX aggregation for 8821au/8812au

USB RX aggregation improves the RX speed on certain ARM systems, like
the NanoPi NEO Core2. With RTL8811AU, before: 30 Mbps, after: 224 Mbps.

The out-of-tree driver uses aggregation size of 7 in USB 3 mode, but
that doesn't work here. rtw88 advertises support for receiving AMSDU
in AMPDU, so the AP sends larger frames, up to ~5100 bytes. With a size
of 7 RTL8812AU frequently tries to aggregate more frames than will fit
in 32768 bytes. Use a size of 6 instead.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/63012163-a425-4b15-b830-43f279c06b73@gmail.com