mac80211_hwsim: report survey data for scanned channels

Currently, hwsim is reporting survey data (only a fake noise floor)
for the current channel. This breaks when the multi-channel support
is enabled since then there's no current channel.

Make the dummy implementation closer to a real one and only report
data while scanning, for all the scanned channels. At other times,
no survey data might be available (in real hardware) due to power-
save for example.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>