mac802154: cleanup ieee802154_netdev_to_extended_addr

This patch cleanups the ieee802154_be64_to_le64 to have a similar
function like ieee802154_le64_to_be64 only with switched source and
destionation types.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>