i2c: Drivers stop using the redundant client list

The redundant i2c client list maintained by i2c-core is going away
soon, so drivers should stop using it now. Instead, they can use the
standard iterator provided by the device driver model
(device_for_each_child).

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: David Brownell <david-b@pacbell.net>
Cc: Michael Hunold <michael@mihu.de>