ixgbe: add support for reporting 5G link speed

Since 5G link speed is supported by some devices, add reporting of 5G link
speed.

Signed-off-by: Paul Greenwalt <paul.greenwalt@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>