iwlwifi: introduce device family enum

This will later be used to dynamically bind
the configuration data for DVM and MVM.

For now, we can use it to get rid of the
additional_nic_config() hook.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>