i2c: add debug info when class instantiation was dropped

We have a warning already when support for old-fashioned class based
instantiation is about to be dropped somewhen soon from a driver. Let's
have another one when it was actually dropped. This allows to remove the
cruft a little earlier and still let users know what happened in the
rare case they are missing devices after the change. However, there is
enough interest to get rid of class based instantiation rather sooner
than later because it improves boot up time.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>