hwmon: (tmp421) 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: Andre Prendel <andre.prendel@gmx.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>