i40e: clean up throttle rate code

The interrupt throttle rate minimum is actually 2us, so
fix that define and while we are there, remove some unused defines.

Change some strings in the function to be a bit less wrappy, and
express the correct limits.

Change-ID: I96829bbc77935e0b57c6f0fc1439fb4152b2960a
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Patrick Lu <patrick.lu@intel.com>
Tested-by: Jim Young <jamesx.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>