100M SGMII is only supported on X722.  Replace the mac check with
a feature flag check that is only set for the X722 device.

Change-ID: I53452d9af6af8cd9dca8500215fbc6ce93418f52
Signed-off-by: NCatherine Sullivan <catherine.sullivan@intel.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>