MLBEDSW-8674: int16 VectorProduct should use Natural rounding

 - Fixed output diff for FullyConnect int16
 - Problem was that wrong rounding mode was used
 - Reference uses Natural rounding for FullyConnect int16

Change-Id: I209313b6f89fed01678a448a935d5f6904b41057
Signed-off-by: Johan Alfven <johan.alfven@arm.com>