i40evf: Fix VLAN features

Users of ethtool were being given the mistaken impression that this
driver was able to change its VLAN tagging features, and were
disappointed that this was not actually the case. Implement
ndo_fix_features method so that we can adjust these flags as needed to
avoid false impressions.

Change-ID: I08584f103a4fa73d6a4128d472e4ef44dcfda57f
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>