i40e: report correct VF link speed when link state is set to enable

When the virtual link state was set to "enable" ethtool would report
link speed as 40000Mb/s regardless of the underlying device.
Report the correct link speed.

Example from a XXV710 NIC.
Before:
$ ip link set ens3f0 vf 0 state auto
$  ethtool enp8s2 | grep Speed
        Speed: 25000Mb/s
$ ip link set ens3f0 vf 0 state enable
$ ethtool enp8s2 | grep Speed
        Speed: 40000Mb/s
After:
$ ip link set ens3f0 vf 0 state auto
$  ethtool enp8s2 | grep Speed
        Speed: 25000Mb/s
$ ip link set ens3f0 vf 0 state enable
$ ethtool enp8s2 | grep Speed
        Speed: 25000Mb/s

Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>