Use throughput estimate-based BSS selection with larger SNR difference

Use throughput estimate-based BSS selection with larger SNR difference

Previously, the est_throughput comparison was done only when SNR
difference was less than 5 dB. Since the throughput estimation take into
account SNR, this can be done in more cases. For now, add a conservative
2 dB more to the difference so that any SNR difference below 7 dB
results in BSS selection based on throughput estimates.

In addition, the throughput estimates require SNR values to be
available, so separate this from the 5 GHz preference that can be done
based on either SNR or qual values.

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