ixgbe: Enable support for recognizing PCI-e Gen3 link speed

This patch adds support for displaying PCIe Gen3 link speed, which was
previously missing from the driver.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>