i40e: don't configure zero-size RSS table

In some rare cases, we might get a VSI with no queues. In this case, we
cannot configure RSS on this VSI as it will try to divide by zero when
configuring the lookup table.

Change-ID: I6ae173a7dd3481a081e079eb10eb80275de2adb0
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>