ice: Calculate ITR increment based on direct calculation

ice: Calculate ITR increment based on direct calculation

Currently when calculating how much to increment ITR by inside of
ice_update_itr() we do some estimations and intermediate
calculations. Instead of doing estimations, just do the
calculation directly. This allows for a more accurate value and it
makes it easier for the next person to understand and update.

Also, remove the dividing the ITR value by 2 when latency
driven because the ITR values are already so low for 100Gbps
speed. This should help get to the desired ITR value faster.

Signed-off-by: Brett Creeley <[email protected]>
Signed-off-by: Anirudh Venkataramanan <[email protected]>
Tested-by: Andrew Bowers <[email protected]>
Signed-off-by: Jeff Kirsher <[email protected]>