ixgbe: Use MDIO_PRTAD_NONE consistently

The value MDIO_PRTAD_NONE should be used to indicate no PHY address.
Not 0, not 0xFFFF. Use the MDIO_PRTAD_NONE value consistently.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>