i2c: Keep client->driver and client->dev.driver in sync

Ensure that client->driver is set to NULL if the probe() returns an
error (this keeps client->driver and client->dev.driver in sync).

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>