i40e: allow XPS with QoS enabled

Recently, the kernel gained support for enabling XPS and QoS at the
same time. Thus, we no longer need to worry about the number of
traffic classes when enabling XPS.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>