802.11n: scan for overlapping BSSes before starting 20/40 MHz channel

802.11n: scan for overlapping BSSes before starting 20/40 MHz channel

Try to match PRI/SEC channel with neighboring 20/40 MHz BSSes per
IEEE 802.11n/D7.0 11.14.3.2. This is not yet complete implementation,
but at least some parts of the 40 MHz coex are improved.

40 MHz operation maybe rejected (i.e., fall back to using 20 MHz) or
pri/sec channels may be switched if needed.