i40evf: don't lie to ethtool

Don't tell ethtool that the VF can do 10GbaseT, when it really has no
idea what its link speed is. Set the supported values to 0 instead.
Suggested by Ben Hutchings.

CC: Ben Hutchings <ben@decadent.org.uk>
Change-ID: Iceb0d8af68fe5d8dc13224366979ba701ba89c39
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>