Use a dedicated bitfield in order to both increase
the amount of checking around the length of ITR writes
as well as simplify the checks of dynamic mode.

Basically unpack the "high bit means dynamic" logic
into bitfields.

Also, remove some unused ITR defines.
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>