mrf24j40: calling ieee802154_register_hw at last

The function ieee802154_register_hw should always called at last.
Currently we do hardware init and such things after register hardware
into the subsystem. It could be that the subsystem already call driver
operations while running hardware init.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>