wcn36xx: Setup starting bitrate to MCS-5

By default, after associated to an AP, the wcn36xx bitrate adjustment
algorithm starts sending data at 1Mbps, and increases the rate slowly
(1Mbps, 2Mbps, 6Mbps...) over the further TX packets.

Starting at 1Mbps usually causes the initial throughput to be really
low and the maximum possible bitrate to be reached after about hundreed
of TX packets.

That can be improved by setting a different initial bitrate for data
packets via the ENABLE_DYNAMIC_RA_START_RATE configuration value, this
value can be a legacy or MCS rate.

This patch sets the starting bitrate value to MCS-5, which seems to be
a good compromise given it can be quickly adjusted low or up if necessary.
(and based on what I observed in the wild with some mobile devices)

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1598345341-4505-1-git-send-email-loic.poulain@linaro.org