i40e: implement and use Rx CTL helper functions

Use the new AdminQ functions for safely accessing the Rx control
registers that may be affected by heavy small packet traffic.

Change-ID: Ibb00983e8dcba71f4b760222a609a5fcaa726f18
Signed-off-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>