ixgbe: correct CS4223/7 PHY identification

Previous method was unreliable. Use a different register to
differentiate between the SKUs.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>