ixgbe: Fix ethtool diag test for VT mode operation

Skip MAC loopback test when the adapter is set to a VT mode such as SR-IOV
or VMDq

Signed-off-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>