i40e/ethtool: support coalesce getting by queue

This patch implements get_per_queue_coalesce for i40e driver.

Signed-off-by: Kan Liang <kan.liang@intel.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>