ieee802154: don't allow to change addr while netif_running

This patch changes the actual behaviour for setting address attributes.
We should not change addresses while netif_running is true. Furthermore
when netif_running is running the address attributes becomes read only
and we can remove locking mechanism in receive and transmit hothpaths
of 802.15.4 subsystem.

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