hwmon: (ltc2990) Fix incorrect conversion of negative temperatures

Fix incorrect conversion of negative temperatures by using
sign_extend32() instead of a home-grown conversion function.

Fixes: df922703 ("hwmon: Add LTC2990 sensor driver")
Signed-off-by: Tom Levens <tom.levens@cern.ch>
[groeck: Updated subject and description]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>