iwlwifi: check control channel for pure 40MHz

for pure 40MHz mode, set the control channel location if provided, but not
like Mixed mode; if information is not provided, still allow 40MHz
operation.

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>