i40e/i40evf: adjust packet size to account for double VLANs

Now that the kernel supports double VLAN tags, we should at least play
nice. Adjust the max packet size to account for two VLAN tags, not just
one.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>