ACS: Accept channel if any (rather than all) survey results are valid

ACS: Accept channel if any (rather than all) survey results are valid

Previously, a channel with even a single scan/survey result missing
information was skipped in ACS. This may not be desirable in cases when
multiple scan iterations are used (which is the case by default in
hostapd). Instead, use all channels that provided at least one complete
set of results. Calculate the average interference factor as an average
of the iterations that did provide complete values.

This seems to help with some cases, e.g., when ath9k may not be able to
report the noise floor for all channels from the first scan iteration
immediately after the driver has been loaded, but then returns it for
all other scan iterations.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>