Ideally, we'd check that it was only the "set" function which was __init,
and that the permissions were r/o.  But that's a little hard.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Phil Carmody <ext-phil.2.carmody@nokia.com>