st/nine: Fix rounding issue with vs1.1 a0 reg

st/nine: Fix rounding issue with vs1.1 a0 reg

vs1.1 rounds a0 to lowest integer, while
other versions do round to closest.

To use the same path as the other versions (with ARR),
we were substracting 0.5 for vs1.1 to get round to lowest.

This gives wrong result if a0 is set to 0:
round(0 - 0.5) = -1

Instead just use ARL for vs1.1

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>