powerpc/ibmebus: Fix further device reference leaks

Make sure to drop any reference taken by bus_find_device() when creating
devices during init and driver registration.

Fixes: 55347cc9

 ("[POWERPC] ibmebus: Add device creation and bus probing based on of_device")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>