ixgbe: Do not read the spoofed packets counter when not in IOV mode

The counter is not valid unless the controller is running in IOV mode.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>