Adjust max bitrate SNR floors

Adjust max bitrate SNR floors

These values were defined in commit a1b790eb9d75 ("Select AP based on
estimated maximum throughput") with no justification. Other sources
[0,1,2] give a different (consistent) set of SNR floors per MCS index.
Adjust the values accordingly.

[0] http://www.revolutionwifi.net/revolutionwifi/2014/09/wi-fi-snr-to-mcs-data-rate-mapping.html
[1] https://higher-frequency.blogspot.com/2016/10/80211n-80211ac-data-rates-and-snr.html
[2] https://www.wlanpros.com/resources/mcs-index-802-11ac-vht-chart/

Signed-off-by: Matthew Wang <matthewmwang@chromium.org>