i40e: Clean up dead code

The function i40e_client_prepare() can never return an error. So make it
void and quit checking its return value.

Change-ID: I9ff311e2324dde329eb68648efb2c94aaff856db
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>