ieee802154: add iftype to wpan_dev

This patch adds an iftype argument to the wpan_dev. This is needed to
get the interface type from netdev ieee802154_ptr. The subif data struct
can only accessible in mac802154 branch.

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