rtwn: set the maximum A-MPDU size correctly for RTL8812AU/RTL8821AU

The vendor driver sets it to 64k or 128k depending upon chipset,
along with bit 31 being set in hal/rtl8812a_hal_init.c:SetHwReg8812A().

Differential Revision: https://reviews.freebsd.org/D48118
Obtained from: https://github.com/lwfinger/rtl8812au
Reviewed by: bz