i40e: Enable VF Tx bandwidth setting

Implement the net device op for Tx bandwidth setting. Setting the Tx
bandwidth is done by 'ip link set <PF device> vf <VF num> rate <Tx
rate>', with the rate specified in Mbit/sec. The rate setting is
displayed with 'ip link show'.

Change-ID: I4d45dda8320632fdb6ec92c87d083e51070b46ab
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Acked-by: Shannon Nelson <shannon.nelson@intel.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>