hwmon: (ad7418) Catch I2C errors

If there is an i2c failure, the ad7416 reports its temperature as 0C.
Return an error code so users can properly detect errors.

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>