ieee802154: rework interface registration

This patch meld mac802154_netdev_register into ieee802154_if_add
function. Also we have now only one alloc_netdev call with one interface
setup routine "ieee802154_if_setup" instead two different one for each
interface type. This patch checks via runtime the interface type and do
different handling now. Additional we add the wpan_dev struct in
ieee802154_sub_if_data and set the new ieee802154_ptr while netdev
registration. This behaviour is very similar the mac80211 netdev
registration functionality.

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