mac802154: move local started handling

mac802154: move local started handling

This patch removes the current handling of started boolean. This is
actually dead code, because mac802154_netdev_register can't never be
called before ieee802154_register_hw. This means that local->started is
always be true when mac802154_netdev_register is called. Instead we
using this now like mac80211 to indicate that an instance of sdata is
running.

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