Use estimated throughput to improve roaming selection

Use estimated throughput to improve roaming selection

Previously, within-ESS roaming was skipped if the selected BSS did not
have a higher signal strength than the current BSS regardless of AP
capabilities. This could result in not moving to a BSS that would
provide higher throughput, e.g., due to larger channel bandwidth or
higher rates (HT/VHT MCS).

Use estimated throughput information from scan result processing to
allow within-ESS roaming if the selected BSS is likely to provide better
throughput even if the current BSS has larger RSSI.

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