ixgbe: Add support for reporting 2.5G link speed

Now that we can do 2.5G link speed, we need to be able to report it.
Also change the nested triadic involved in creating the log message
to instead use a simpler switch statement to set a string pointer.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>