i2c: Discard warning message on device instantiation from user-space

The "new_device" sysfs interface has been there for quite some time
now, nobody complained about it so it must be good enough. Time to
remove the warning and call it stable.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Michael Lawnick <ml.lawnick@gmx.de>