kunit: unregister the device on error

kunit_init_device() should unregister the device on bus register error,
but mistakenly it tries to unregister the bus.

Unregister the device instead of the bus.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Fixes: d03c720e ("kunit: Add APIs for managing devices")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>