i2c-dev: attach/detach_adapter cleanups

* Only print that an adapter was attached when it succeeds.
* i2c_dev == NULL on detach simply means that the attach failed
  before, this isn't an error per se.
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>