i40e/i40evf: Fix and refactor dynamic ITR code

This patch changes the switch statement for dynamic interrupt throttling
and adds a default case. With this patch, we check the latency setting
instead of the current ITR settings and the included refactor improves
performance.

Without this patch, the ITR setting would never change dynamically, and
there was no default.

Change-ID: Idb5a8a14c7109ec47c90f6e94bd43baa17d7ee37
Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>