i40e/i40evf: Use dma_rmb where appropriate

Update i40e and i40evf to use dma_rmb.  This should improve performance by
decreasing the barrier overhead on strong ordered architectures.

Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>