Make it a bit easier to roam from 2.4 GHz to 5 GHz within ESS

Make it a bit easier to roam from 2.4 GHz to 5 GHz within ESS

The initial connection to an ESS was already explicitly increasing the
likelihood of picking a 5 GHz BSS. While the throughput estimation is
likely to do same for the roaming decision, it might be possible that
that does not cover all cases. Add couple of dB extra preference for 5
GHz in case the roaming decision falls back to comparing signal levels.

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