hwmon: (amc6821) Fix return value

Propagate return value obtained from i2c_smbus_read_byte_data()
instead of hardcoding.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: T. Mertelj <tomaz.mertelj@guest.arnes.si>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>