igb: Implementation to report advertised/supported link on i354 devices

This patch changes the way we report supported/advertised link for i354
devices, especially for 2.5 GB. Instead of reporting 2.5 GB for all i354
devices erroneously, check first, if it is 2.5 GB capable.

Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>