i2c: Stop using i2c_adapter.class_dev

Stop using i2c_adapter.class_dev, as it is going to be removed
soon. Luckily, there are only 4 RTC drivers affected.

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