i40e: remove and fix confusing define name

I40E_ITR_NONE was being used as an ITRN register index by
accident because it was easily associated with the I40E_RX_ITR
and friends defines.

Change the name slightly in order to make it clear that
I40E_ITR_NONE is really associated with the DYN_CTL register
sets.

Change-Id: I04702c027c7495b90a8bf2db85d3e085a2c7d02a
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>