i2c: Improve logging on failure to probe for ->class devices

While writing the i2c-imc driver, I noticed that the warning message
when the i2c core can't figure out how to probe is mostly useless.
This trivial patch improves it.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>