mrf24j40: use ieee802154_alloc_hw for private data

This patch removes the own private dataroom allocation by calling
devm_kzalloc for devrec and assign this pointer to "devrec->hw->priv".
Instead we using like all other drivers ieee802154_alloc_hw and give the
size for the private driver dataroom at the first argument.

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>