hwmon: (ad7418) Convert to a new-style i2c driver

The ad7418 driver is only used on embedded platforms where i2c
devices can easily be declared in platform code. Thus a new-style
i2c driver makes perfect sense. This lets us get rid of quirky
detection code (these chips have no identification registers) and
shrinks the binary driver size by 38%.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>