Driver core: Fix error handling in bus_add_driver().

- If the allocation of ->priv fails, the reference on the bus
  must be dropped.
- If adding the kobject fails, kobject_put must be called to
  clean things up.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>