Select AP based on estimated maximum throughput

Select AP based on estimated maximum throughput

This modifies the BSS selection routines to calculate SNR and estimated
throughput for each scan result and then use the estimated throughput as
a criteria for sorting the results. This extends the earlier design by
taking into account higher throughput rates if both the AP and local
device supports HT20, HT40, or VHT80. In addition, the maximum rate is
restricted based on SNR.

In practice, this gives significantly higher probability of selecting
HT/VHT APs when there are multiple BSSes in the same ESS and SNR is not
low enough to prevent higher MCS use.

Signed-off-by: Jouni Malinen <j@w1.fi>