i40e: fix double 'NIC Link is Down' messages

When isup is false meaning that interface is going to shut down
set new speed to 0 to avoid double 'NIC Link is Down' messages.

Signed-off-by: Sergey Nemov <sergey.nemov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>