wcn36xx: Fix (QoS) null data frame bitrate/modulation

We observe unexpected connection drops with some APs due to
non-acked mac80211 generated null data frames (keep-alive).
After debugging and capture, we noticed that null frames are
submitted at standard data bitrate and that the given APs are
in trouble with that.

After setting the null frame bitrate to control bitrate, all
null frames are acked as expected and connection is maintained.

Not sure if it's a requirement of the specification, but it seems
the right thing to do anyway, null frames are mostly used for control
purpose (power-saving, keep-alive...), and submitting them with
a slower/simpler bitrate/modulation is more robust.

Cc: stable@vger.kernel.org
Fixes: 512b191d ("wcn36xx: Fix TX data path")
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1634560399-15290-1-git-send-email-loic.poulain@linaro.org