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>