i40e: Define and use i40e_is_vf macro

This patch is useful for future expansion when new VF MAC types get
added. It helps with cleaning up VF driver flow.

Change-ID: Ibe1eeb71262a3a40f24a1c5409436bdc3411da7f
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Acked-by: Shannon Nelson <shannon.nelson@intel.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>