mac80211: filter scan results on unusable channels

When you have an AP on channel 13, it will currently often enough
be listed in scan results even when the regulatory domain restricts
to channels 1-11. This is due to channel overlap. To avoid getting
very strange failures, don't show such APs in the scan results. The
failure mode will now go from "I can see the AP but not associate"
to "I can't see the AP although I know it's there" which is easier
to debug.

This problem was first really noticed by Jes Sorensen.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Jes Sorensen <jes@trained-monkey.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>