MLBEDSW-10837: Improve Ethos-U85 block config search ratio

OFMs with extreme aspects (like 1:200) end up using
a pathological scaling ratio during the block config
search.
 - This commit changes the scaling ratio to be more
   conservative thus refining more accurately  (at the
   cost of speed).
 - Fixed issue where the last, smallest-possible
   configuration was not selectable.

Signed-off-by: Philip Hall <philip.hall@arm.com>
Change-Id: I20eed6268543453267c76f057b1a84b7cbadc1c5