tcg/i386: fix unsigned vector saturating arithmetic

Due to a cut/paste error in the original implementation, the unsigned
vector saturating arithmetic was erroneously being calculated as signed
vector saturating arithmetic.

Fixes: 8ffafbce ("tcg/i386: Implement vector saturating arithmetic")
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20190207224258.426-1-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>