ieee802154: 6lowpan: remove set of mac address

Currently the ieee802154 6lowpan interface operates on wpan interfaces
only. Setting the wpan mac address over 6lowpan interface is complex and
maybe we can't never do this. This patch removes the set of mac address
handling in ieee802154 6lowpan interface for now.

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