i40e: fix 'Unknown bps' in dmesg for 2.5Gb/5Gb speeds

This patch fixes 'NIC Link is Up, Unknown bps' message in dmesg
for 2.5Gb/5Gb speeds. This problem is fixed by adding constants
for VIRTCHNL_LINK_SPEED_2_5GB and VIRTCHNL_LINK_SPEED_5GB cases
in the i40e_virtchnl_link_speed() function.

Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>