Use signal quality if level is not available for comparing max rates

Use signal quality if level is not available for comparing max rates

Some drivers (for example ipw2100) do not report signal level but only
signal quality. wpa_supplicant already uses the signal quality if no
level is reported and all other comparision parameters are equal to sort
the scan results. However, if two APs have different max rates and the
signal level does not differ much wpa_supplicant chooses the AP with the
higher max rate.

In case of ipw2100 no signal level is reported and thus wpa_supplicant
always takes the AP with higher max rate even if its signal quality is
really low. For example if AP1 (max rate 11Mb/s, 80% signal quality) and
AP2 (max rate 54 Mb/s, 20% signal quality) are found by a scan
wpa_supplicant would choose AP2.

Hence, if no signal level is reported depend on the signal quality if
max rate should be compared. A quality difference of 10% is considered
acceptable in favor of the higher max rate.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>