igbvf: work around i350 erratum

On i350 VF devices, VLAN tags will be byte-swapped in the receive
descriptor only when received packets are looped back from other
VFs. Check for this condition and swab the tag if needed.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>