ixgbe: Return error when getting PHY address if PHY access is not supported

In cases where PHY register access is not supported, don't mislead
a caller into thinking that it is supported by returning a PHY
address. Instead, return -EOPNOTSUPP when PHY access is not
supported.

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