iwlwifi: 8000: add 11n only SKU of 8000 devices

The 8000 family includes devices that don't support 11ac.
Add an iwl_cfg structure for them.

Signed-off-by: Ariej Marjieh <ariej.marjieh@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>