i2c-viapro: Don't log nacks

Transactions not acked can happen every now and then, in particular
during device detection, and various transaction types can be used for
this purpose. So stop logging this event, except when debugging is
enabled. This is what other similar drivers (e.g. i2c-i801 or
i2c-piix4) do.

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