i2c: Remove the warning on missing adapter device

Now that the i2c_adapter migration plan changed and we are going to
keep i2c_adapter.dev, it's no longer that urgent to add a proper device
to all i2c_adapter drivers. Thus is seems resonable to degrade the
warning asking authors to migrate their driver to a debug message.

Signed-off-by: Jean Delvare <khali@linux-fr.org>