i40e: make prep_for_reset void

The return from i40e_prep_for_reset() was being ignored by almost all
its callers.  The one place it wasn't ignored could have caused a silent
and confusing failure of the driver to finish a reset.  Since we really
are doing a rebuild anyway, ignore this last case as well and simply
make the function a void type.

Change-ID: Ia4fed7f903d39a6c47c5722625a53e59c3f7ed53
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>