i40e: don't allow changes to HW VLAN stripping on active port VLANs

Modifying the VLAN stripping options when a port VLAN is configured
will break traffic for the VSI, and conceptually doesn't make sense,
so don't allow this.

Signed-off-by: Nicholas Nunley <nicholas.d.nunley@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>