hwmon: (adc128d818) Do proper sign extension

data->temp[index] has type s16. Because of C's promotion rules,
(data->temp[index] << 7) >> 7 is exactly the same as
data->temp[index]. The intention was to use bit 8 as a sign bit, so do
that using the existing API.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>