ixgbevf: Enable GRO by default

Enable GRO by default for performance.

Signed-off-by: Shirley Ma <xma@us.ibm.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>