i2c: Don't uselessly set i2c_adapter.retries

I2C adapter drivers are supposed to handle retries on nack by themselves
if they do, so there's no point in setting .retries if they don't.

As this retry mechanism is going away (at least in its current form),
clean this up now so that we don't get build failures later.

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