i2c-powermac: Be less verbose in the absence of real errors.

Be less verbose in the absence of real errors. We don't have to report
failed probes to the users, it's only confusing them.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Tested-by: Andrey Gusev <ronne@list.ru>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: stable@kernel.org