mac802154: add ieee802154_vif struct

This patch adds an ieee802154_vif similar like the ieee80211_vif which
holds the interface type and maybe further more attributes like the
ieee80211_vif structure.

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