ixgbe: advertise highest capable link speed

On module insert advertise highest capable link speed. If module is
capable of 10G, then advertise 10G, else advertise modules capable
link speeds.

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>