hwmon: (amc6821) sign extension temperature

Converts the unsigned temperature values from the i2c read
to be sign extended as defined in the datasheet so that
negative temperatures are properly read.

Fixes: 28e6274d ("hwmon: (amc6821) Avoid forward declaration")
Signed-off-by: Jared Bents <jared.bents@rockwellcollins.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
[groeck: Dropped unnecessary continuation line]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>