i40evf: support queue-specific settings for interrupt moderation

In commit a75e8005

 ("i40e: queue-specific settings for interrupt
moderation") the i40e driver gained support for setting interrupt
moderation values per queue. This patch adds support for this feature
to the i40evf driver as well. In addition, a few changes are made to
the i40e implementation to add function header documentation comments,
as well.

This behaves in a similar fashion to the implementation in i40e. Thus,
requesting the moderation value when no queue is provided will report
queue 0 value, while setting the value without a queue will set all
queues at once.

Change-ID: I1f310a57c8e6c84a8524c178d44d1b7a6d3a848e
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>