iwlwifi: fix 40MHz operation setting on cards that do not allow it

Some devices have 40MHz operation disabled entirely. Ensure that driver do
not enable 40MHz operation if a channel does not allow this.

This fixes http://bugzilla.intellinuxwireless.org/show_bug.cgi?id=2135



Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
CC: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>