i40e: queue-specific settings for interrupt moderation

For i40e driver, each vector has its own ITR register. However, there
are no concept of queue-specific settings in the driver proper. Only
global variable is used to store ITR values. That will cause problems
especially when resetting the vector. The specific ITR values could be
lost.
This patch move rx_itr_setting and tx_itr_setting to i40e_ring to store
specific ITR register for each queue.
i40e_get_coalesce and i40e_set_coalesce are also modified accordingly to
support queue-specific settings. To make it compatible with old ethtool,
if user doesn't specify the queue number, i40e_get_coalesce will return
queue 0's value. While i40e_set_coalesce will apply value to all queues.

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