i40e: register existing client on probe

In some cases, a client (i40iw) may already be present when probe is
called. Check for this, and add a client instance if necessary.

Change-ID: I2009312694b7ad81f1023919e4c6c86181f21689
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>