rtl8xxxu: Improve the A-MPDU retransmission rate with RTS/CTS protection

The A-MPDU TX retransmission rate is always high (> 20%) even in a very
clean environment. However, the vendor driver retransimission rate is
< 10% in the same test bed. The difference is the vendor driver starts
the A-MPDU TXOP with initial RTS/CTS handshake which is observed in the
air capture and the TX descriptor. Since the driver does not know how
many frames will be aggregated and the estimated duration, forcing the
RTS/CTS protection for A-MPDU helps to lower the retransmission rate
from > 20% to ~12% in the same test setup with the vendor driver.

Signed-off-by: Chris Chiu <chris.chiu@canonical.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20211215085819.729345-1-chris.chiu@canonical.com