iwlwifi: debugFs to enable/disable HT40 support

Add debugfs file to enable/disable HT40(40MHz) channel support.
By default, 40MHz is supported if AP can support the function.

By echo "1" to "disable_ht40" file, iwlwifi driver will disable the
40MHz support and only allow 20MHz channel.

Because the information exchange happen during association time,
so enable/disable ht40 channel only can be performed when it is not
associated with AP.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>