i40e: refactor DCB function

This is a simple refactor suggested by the community
to change a multi-level if statement into a switch.

Change-ID: I831cf3c40426022220aa9b43990022d22dfd50db
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>