mac802154: introduce IEEE802154_DEV_TO_SUB_IF

This function adds a wrapper to call netdev_priv to getting the sdata
attribute. This is similar like the IEEE80211_DEV_TO_SUB_IF function
inside wireless stack implementation.

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