i40e/i40evf: Use ring pointers to clean up _set_itr_per_queue

This change cleans up the i40e/i40evf_set_itr_per_queue function by
dropping all the unneeded pointer chases. Instead we can just pull out the
pointers for the Tx and Rx rings and use them throughout the function.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>