iwlwifi: disable the buggy chain extension feature in HW

This feature has been reported to be buggy and enabled by
default. We therefore need to disable it manually.

Cc: stable@vger.kernel.org
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>