iwlwifi: disable coex until implementation ready for 6x50

Temporary disable the coex function for wifi/wimax for 6x50
series until the full implementation ready.

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>