i40e: add 20G speed for Tx bandwidth calculations

When calculating TX bandwidth for VFs, we need to know the link speed to
make sure we don't allocate more bandwidth than is available. Add 20G
link speed to the switch statement so we can support devices that link
at that speed.

Change-ID: I5409f6139d549e5832777db9c22ca0664e0c5f8b
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>