MLBEDSW-7578: Fix output diff caused by wrong rounding in Conv2d

- The reference calculates the rounding different between
int8 and int16 for Conv2d. However, internally a Conv2d can be
changed to a FullyConnect but then the rounding must still be
calculated following the Conv2d reference.

- The fix is to check the original type if NATURAL rounding
should be used or not. int16 Conv2d uses NATURAL rounding
in reference.

Change-Id: I80d48b54372ef7b978ee2e9384a01934dd454e24
Signed-off-by: Johan Alfven <johan.alfven@arm.com>