igb: add support for reporting 5GT/s during probe on PCIe Gen2

This change corrects the fact that we were not reporting Gen2 link speeds
when we were in fact connected at Gen2 rates.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>