ieee802154: cleanups for ieee802154.h

This patch removes some const from non-pointer types and fixes the
function name for the ieee802154_is_valid_extended_unicast_addr
comment.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>