mac802154: main: move open and close into iface

These functions can be static inside the iface file, because it's not
used anywhere else. This patch moves these functions into iface file.

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