mac820154: move interface unregistration into iface

This patch move the iface unregistration into iface.c file to have
a behaviour which is similar like mac80211. Also iface handling should
be inside iface.c file only.

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

diff --cc net/mac802154/ieee802154_i.h
Simple merge

diff --cc net/mac802154/iface.c
Simple merge

diff --cc net/mac802154/main.c
Simple merge