i40e: only register client on iWarp-capable devices

The client interface is only intended for use on devices that support
iWarp. Only register with the client if this is the case.

This fixes a panic when loading i40iw on X710 devices.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Reported-by: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>