i40e: fix curly brace use and return type

Add curly-braces on a multi-line function.  While we're here we
also change to return void in i40e_vsi_clear_rings() since no
caller cares.

Change-Id: I261fcef20e2a39e18d83ec08fdd14456131dee91
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>