rtl8xxxu: Use lower tx rates for the ack packet

According to the Realtek propritary driver and the rtw88 driver, the
tx rates of the ack (includes block ack) are initialized with lower
tx rates (no HT rates) which is set by the RRSR register value. In
real cases, ack rate higher than current tx rate could lead to
difficulty for the receiving end to receive management/control frames.
The retransmission rate would be higher then expected when the driver
is acting as receiver and the RSSI is not good.

Cross out higer rates for ack packet before implementing dynamic rrsr
configuration like the commit 48308726 ("rtw88: add dynamic rrsr
configuration").

Signed-off-by: Chris Chiu <chris.chiu@canonical.com>
Acked-by: Jes Sorensen <Jes.Sorensen@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211001040044.1028708-1-chris.chiu@canonical.com