libertas_tf: fix signal reporting

Instead of exposing the signal-to-noise ration, calculate the actual signal
level taking the noise floor into account.

Also, flip the SIGNAL_DBM bit on, so that mac80211 exposes the signal
level along with the station info in scan results. This fills
NetworkManager's "nmcli d wifi output" output with colors, bars and joy.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Steve deRosier <derosier@cal-sierra.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>