MLBEDSW-8873: MLCE: Update LUT index calculation

 - A network containing several softmax operators caused an
output diff
 - The problem was that the code that detects if the LUT is
already in internal SRAM calculated everything correctly except
for which lut index to use.
 - The code should use the slot_size and not then LUT size when
calculating the index which fixes this problem.
 - Updated unit tests

Change-Id: I07686651a883ccbba7c173e7191eb21f9ff15bf5
Signed-off-by: Johan Alfven <johan.alfven@arm.com>