iwlwifi: mvm: add and use iwl_mvm_has_unified_ucode()

This may need to be refined later, but for now using this,
even with the TODO, is better than checking "has new TX API".

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>