i40e/i40evf: Remove reference to ring->dtype

As part of the rx-refactor, the dtype variable in the i40e_ring
struct is no longer used, so remove it.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>