driver core: Remove device_is_registered() in device_move().

device_is_registered() will always be false for a device with no bus. Remove
this check and trust the caller to know what they're doing.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>