i40e: Warn when setting link-down-on-close while in MFP

This patch adds a warning message when the link-down-on-close flag is
setting on. The warning is printed only on MFP devices

Signed-off-by: Paweł Jabłoński <pawel.jablonski@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>