i40e/i40evf: Only offload VLAN tag if enabled

The driver was offloading the VLAN tag into the skb
any time there was a VLAN tag and the hardware stripping was
enabled.  Just check to make sure it's enabled before put_tag.

Change-Id: Ife95290c06edd9a616393b38679923938b382241
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>