hwmon: (jc42) Use sign_extend32 for sign extension

Despite the name, sign_extend32 works just fine for 16 bit variables,
so it is safe to use.

Cc: Martin Kepplinger <martink@posteo.de>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>