iwlwifi: mvm: use CTS to Self if firmware allows it

Newer firmware fixed a bug that prevented to use CTS to
self. Firmwares with API greater than 8 have this bug
fixed. Enable the feature for these firmwares only.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>