cfg80211: allow drivers to selectively disable 80/160 MHz

Some drivers might support 80 or 160 MHz only on some
channels for whatever reason, so allow them to disable
these channel widths. Also maintain the new flags when
regulatory bandwidth limitations would disable these
wide channels.

Reviewed-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>