mac80211: allow disable FAT in specific configurations

This patch allows to disable FAT channel in specific configurations.

For example the configuration (8, +1), (primary channel 8, extension
channel 12) isn't permitted in U.S., but (8, -1), (primary channel 8,
extension channel 4) is. When FAT channel configuration is not
permitted, FAT channel should be reported as not supported in the
capabilities of the HT IE in association request. And sssociation is
performed on 20Mhz channel.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>