ixgbevf: Enable jumbo frame support for X540 VF

The X540 controller allows jumbo frame setup on a per VF basis.  Enable
use of jumbo frames when the VF device belongs to the X540 controller.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>